Escherichia coli Mastitis in Dairy Cattle: Etiology, Diagnosis, and Treatment Challenges

1Bovine mastitis is an inflammation of the udder tissue parenchyma that may result in physical, chemical, and bacteriological changes in milk, and pathological alterations in the glandular tissue. Although mastitis can be caused by multiple events, e.g., traumatic, physiological, allergic, etc., infectious events are the most prevalent, as more than 140 species of microorganisms are involved in bovine mastitis. The disease results in significant financial losses owing to diagnostic tests, veterinary services, wasted milk and labor, and indirect expenses such as low reproduction rates and premature culling of affected animals. Mastitis-related economic losses in the United States are projected to be USD 2 billion annually, with each mastitis case costing producers an average of USD 326. In Israel, Holstein cows diagnosed with clinical mastitis caused by Escherichia coli had a decrease in daily milk yield of almost 15%, which is considered an average milk loss of about 200 liters per cow during the 305 days of the lactation period. In persistent infections, the loss of production is significantly higher; it is estimated at 1,500 liters of milk per cow with a decrease in daily production greater than 30%. In addition to its effects on dairy cattle, mastitis poses a threat to human health due to the risk of transmitting zoonotic pathogens through ingestion of contaminated milk or direct contact with infected cattle. Importantly, the emergence of antibiotic-resistant bacteria in milk and dairy products due to the indiscriminate use of antibiotics to treat bovine mastitis is a serious concern. Notably, E. coli is a leading cause of acute clinical mastitis in dairy cattle worldwide.

2Escherichia coli comprises a highly heterogeneous group of commensal residents of the gut; however, because of the flexibility of its genome, this organism has evolved into pathogenic strains capable of causing diseases, including bovine mastitis following fecal contamination of the teat skin. Mammary pathogenic E. coli (MPEC) has been proposed as a new pathotype responsible for causing mastitis in dairy animals. The mammary gland is not a natural or primary habitat for E. coli because of the milk-associated innate immunity components, such as antimicrobial peptides, lysozyme, lactoferrin, and complement. However, some E. coli have acquired specific virulence factors (VFs) that might help the bacteria to invade the mammary gland, survive, and multiply in milk, turning the mammary gland into an amenable opportunistic habitat for these bacteria. Given the relevance of mastitis caused by E. coli, this paper reviews the important aspects of the disease, the characteristic of the etiologic agent E. coli, diagnosis, and treatments.

3Based on disease symptoms, mastitis can be categorized into two different types, including clinical and subclinical. Clinical mastitis is characterized by visible abnormalities in the udder and milk. Physical examination of the affected mammary gland may reveal redness and pain upon palpation, swelling (edema), and induration (hardening). Systemic symptoms and behavioral changes, including fever, anorexia, depression, lethargy, and a reduction in grooming might also occur. Cattle with clinical mastitis are also affected by poor milk quality, as evidenced by increased somatic cell count (SCC) and compositional changes in milk. SCC is higher compared to the typical counts of under 200,000 cells/mL. The compositional changes in milk include the reduction of lactose content, but little effect on fat and protein contents was observed. Clots and flakes can appear in milk, as well as clear serum-like or bloody discharges. From 1996 to 2014, the reported incidence of clinical mastitis in United States dairy farms increased from 13% (USDA, 1996) to 25%. E. coli is one of the most common causes of clinical bovine mastitis, usually associated with acute symptoms such as dysstasia, diarrhea, and cool extremities. E. coli causes predominantly acute infections, of short duration (10–30 days), mainly in the last 2 weeks of the dry period and in the first weeks postpartum, although reinfections may occur throughout lactation. In most cases, E. coli is eliminated by the host immune response. However, severe cases with systemic involvement are difficult to treat and have a reserved prognosis.

4Subclinical mastitis rarely poses an immediate threat to the animal’s life and is 15–40 times more common than clinical mastitis. This form of the disease cannot be detected by visual inspection of the udder or milk since both appear normal. Consequently, due to the concealed nature of the disease, dairy farmers are unaware of the extent of milk quality loss and the risk of subclinical mastitis spreading to other cows in the herd. Subclinical mastitis is mostly caused by gram-negative bacteria, such as E. coli, Klebsiella pneumoniae, and Serratia marcescens. These pathogens can be transmitted from an infected quarter to an uninfected quarter within the same animal or a different cattle by milking machine (e.g., teat cup liners), dirty milkers’ hands or gloves, and udder washcloths during the milking process. Subclinical mastitis is the most important intramammary illness in cattle, and it costs the dairy farmer more than clinical mastitis due to the dramatic decrease in milk production and quality.

5Extraintestinal pathogenic E. coli (ExPEC) include different sub-groups, e.g., uropathogenic E. coli (UPEC), meningitis-associated E. coli (MNEC), and sepsis-associated E. coli (SEPEC), causing urinary tract infection, meningitis, peritonitis, and septicemia, respectively. This E. coli pathotype-based classification is based on the site of infection, symptoms of the disease, and types of VFs. A novel ExPEC pathotype known as MPEC, one of the most common etiologic agents of bovine mastitis, has been suggested. Compared to other bacteria like Campylobacter spp., Clostridium spp., and Ruminococcus spp., MPEC have undergone evolutionary adaptations (e.g., acquisition of VFs) that improve its colonization of the unique environmental niches found within the mammary gland. These niches include competing microbes, soluble and cellular antimicrobials in milk, e.g., lactoferrin and lactoperoxidase, and the innate response elicited by immune cells. Consequently, E. coli with specific VFs selected within the mammary gland are better suited to cause bovine mastitis.

6Some research studies found that E. coli isolated from cows with mastitis is less genotypically diverse than the environmental strains, and they mostly lack known E. coli VFs. This lack of VFs not only reflects the genome plasticity of E. coli but, most importantly, it suggests that mastitis-associated E. coli may have undergone some selective pressure based on the presence of specific VFs associated with the ability to survive in milk. However, despite recent publications focusing on E. coli genomes isolated from mastitis-affected cattle, researchers were unable to agree upon a common genetic group of putative VFs to all MPEC isolates. Importantly, genome studies on mastitis-associated E. coli have been questioned by some investigators, who argue the need to simultaneously analyze the host-pathogen interaction. For example, it is important to associate physiological traits with genomic data, including techniques such as dual RNA-Seq of the host and bacteria, Tn-Seq to test virulence association of genes in vivo, comparative SNP analysis of orthologous genes and intergenic regions, proteomics, and metabolomics.

7Several research studies suggest that E. coli isolated from mastitis-affected cattle are naturally occurring commensals from the gastrointestinal tract of bovine, mostly from phylogroups A and B1. In other words, it is more likely that E. coli’s VFs implicated in bovine mastitis have their principal function in the colonization and survival in the highly diverse gastrointestinal environment. Consequently, E. coli isolated from mastitis-affected cattle are more likely to be opportunistic and might infect only cattle with a deficient immune system, particularly during the peripartum when leukocyte recruitment to the mammary gland is reduced. In this case, environmental variables and the cattle predispositions (e.g., innate immune system related to the health status, stage of lactation, and parity) are more important than the invading E. coli’s VFs in determining the severity of mastitis.

8Table 1 shows studies on different virulence genes and VFs examined in E. coli isolates from bovine mastitis. Various virulence-related features have been suggested to explain the pathogenicity of E. coli in bovine mastitis, including but not limited to the activation of the innate immune system by pathogen-associated molecular patterns (PAMPs), resistance to serum and neutrophil killing, adhesion and invasion of mammary epithelial cells, and survival and proliferation in milk though secretion system. However, it is important to note that although the aforementioned virulence-associated properties benefit E. coli to thrive in the mammary gland environment, they are not unique to mastitis-associated E. coli, but are also common in ExPEC. Moreover, E. coli strains isolated from cattle with mastitis usually do not present the same virulence characteristics, demonstrating the high genetic abundance and variability of mastitis-causing E. coli.

9Interestingly, the iron scavenging by the ferric dicitrate system has been shown to be strongly associated with mastitis-associated E. coli. The Fec system is essential for the growth of E. coli in milk and may influence disease severity in infections. Iron in bovine milk is limited and kept at quantities below those required to enable E. coli growth because most iron is attached to citrate and a lesser amount to lactoferrin, transferrin, xanthine oxidase, and certain caseins. To overcome the low amount of free iron in milk, E. coli expresses a variety of iron transport systems, such as the high-affinity siderophores (e.g., enterobactin, aerobactin, ferrichrome) and iron-regulated outer membrane proteins (IROMP) that bind to ferric siderophore complexes for transportation into the bacterial cell. In a population-level genomic analysis, showed that the Fec system is required for the pathogenicity of E. coli in the udder environment, as the prototypical MPEC strain P4 was unable to induce mastitis without the system, and the non-mammary gland-pathogenic K71 strain gained MPEC capability with its addition. Importantly, while additional research is needed to fully comprehend the significance of the Fec system in mastitis-associated E. coli, this critical structural component is a promising target for novel chemotherapeutics, enhanced mastitis therapy medicines, and novel vaccines.

10The impact of known virulence genes on the severity of bovine mastitis caused by E. coli is currently unclear, and numerous virulence gene profiles have been found in both persistent and transient isolates, suggesting a high genotypic variability of mastitis-associate E. coli. Importantly, clinical symptoms of E. coli mastitis may be related to genes that have yet to be discovered or to characteristics related to the host (e.g., stage of lactation, number and function of circulating neutrophils, SCC, age of the cattle, nutritional and metabolic status, and genetic resistance). However, persistent infections seem to be related to particular traits, especially the ability of E. coli to transfer from milk to mammary epithelial cells and survive intracellularly. Epithelial cell adhesion and invasion are important events in chronic or persistent infection. In vitro studies demonstrated that persistent mastitis-associated E. coli strains have a higher ability to adhere, invade and survive in bovine mammary epithelial cells line MAC-T than transient strains, are more capable of penetrating an endosome-like compartment of mammary epithelial cells, and have a different intracellular trafficking mechanism than transient strains. Moreover, it was shown that persistent E. coli isolates were more likely to carry lpf genes (encodes for long polar fimbriae), which correlates with E. coli’s ability to invade mammary epithelial cells. Interestingly, in a host-pathogen gene expression study conducted by, both acute and persistent E. coli mastitis strains boosted immune response gene expression (e.g., IRF1, CD83, IL-1α, IL-6, IL-8, CCL20, CXCL 2, MHC 1, Casp 3, CFB), but the immune response increase was substantially greater in cells cocultured with the acute E. coli strain than in cells cocultured with the persistent strain. Similarly, other investigations reported greater up-regulation in immune response functions, inflammation, and antigen processing and presentation in the acute phase than in the chronic phase of bovine mastitis. The induction of a modest immune response by persistent strain is insufficient to eradicate the infection, allowing E. coli to survive in the mammary gland for a lengthy period.

11Bovine innate immunity comprises physical barriers at the tip of the teat and elements such as neutrophils, macrophages, cytokines, natural killer (NK) cells, lactoferrin, and complement, that act predominantly in the early stages of E. coli infections. This variety of immune cell types constitutes the somatic cells (SC), with neutrophils being the predominant type at the beginning of inflammation, representing about 90% of the SC increase. When pathogens invade the mammary gland, they face the first line of cellular defense, such as phagocytosis by neutrophils, which usually respond promptly to the inflammatory process. As a result, the severity of mastitis is determined by the speed with which neutrophils are recruited to the site of infection and how effectively this cell type phagocytes the pathogen. Importantly, the efficacy of the innate response influences the incidence of new intramammary infections, clinical severity, and duration of the case. Another crucial component of the host’s innate immunity is lactoferrin, an iron-tropic glycoprotein with bacteriostatic properties produced by epithelial cells and leukocytes. In ruminants, lactoferrin has been shown to inhibit the multiplication of E. coli by binding to iron ions, making them unavailable for bacterial growth. Additionally, cytokines including interleukins (IL), colony-stimulating factor (CSF), interferon (IFN), and tumor necrosis factor (TNF) are crucial in the mammary gland innate defense. Cytokines are responsible for local signs of inflammation such as swelling, redness, pain, and systemic symptoms such as fever, tachycardia, increased respiratory rate, anorexia, and depression. When innate immunity is not efficient in fighting the infection, the specific (or acquired) immunity mediated by lymphocytes occurs. In response to the inflammation, antibodies produced by lymphocytes pass from the bloodstream to the milk due to increased vascular permeability of the gland. The inflammatory response to mastitis aims to eradicate the microorganism responsible for the infection, neutralize the pathogen’s toxins, and then regenerate injured udder tissue so that the volume of milk normally produced is quickly recovered.

12The onset of mastitis occurs when MPEC penetrates through the teat canal, multiplies in the teat and gland cisterns, and spreads throughout the milk-producing glandular tissue (Figure 1A). Once inside the mammary gland, MPEC ferment lactose and use it as an energy source to multiply without adhering to the gland’s epithelial surface, remaining in the cisterns, ducts, and alveolar lumen. Some MPEC strains can adhere to the udder epithelium, causing chronic intramammary infections and recurrent cases of clinical mastitis. The multiplication and lysis of MPEC release LPS from the bacteria’s outer membrane (Figure 1B). LPS binds to the LPS binding protein (LBP), and the LPS-LBP complex binds to the cluster of differentiation (CD)-14, which leads the LPS to a toll-like receptor-4 (TLR) on the surface of macrophages (;; Figure 1C). When LPS binds to its respective TLR, an intracellular signaling cascade is activated, including the activation of nuclear factor kappa-beta (NF-κβ), which induces macrophages and mammary epithelial cells to synthesize pro-inflammatory cytokines [e.g., interleukin (IL)-1β, IL-1, IL-6, IL-8, and tumor necrosis factor (TNF-α)] and acute-phase proteins (e.g., serum amyloid A and haptoglobin) (;;;;;;;; Figure 1D). Importantly, TNF-α is the main mediator of endotoxic shock that occurs in cases of super-acute mastitis caused by coliforms.

13Chemotactic factors, especially IL-8, recruit neutrophils from the bloodstream that function as phagocytes at the site of infection in the alveoli (; Figure 1E). The efficiency of neutrophils’ diapedesis and phagocytosis directly influence the evolution of the local inflammatory process and the severity of mastitis. Although opsonization is not essential for phagocytosis, immunoglobulins (Ig) such as IgG2 and IgM recognize MPEC through Fab-regions and bind to neutrophils via Fc-receptors on the plasma membrane, leading to phagocytosis in a “zipper mechanism”. The exposure of neutrophils to fat globules and casein leads to loss of cytoplasmic granules (reduced bactericidal activities) and altered morphology (rounding), eliminating pseudopods needed for phagocytosis (;;;; Figure 1F). Consequently, many neutrophils are necessary to cope with the reduced phagocytic capabilities and control the infection. During phagocytosis, neutrophils release chemicals, e.g., reactive oxygen species (ROS) and proteases, that eliminate MPEC but also injure mammary epithelial cells, resulting in a permanent decrease in milk production (;; Figure 1F). To minimize mammary tissue damage, neutrophils undergo programmed cell death (apoptosis) and release chemokines that attract macrophages to the site of infection (; Figure 1G). In the last step of the inflammatory reaction, macrophages quickly phagocytose apoptotic neutrophils (efferocytosis), minimizing the release of granular contents that damage the tissue (; Figure 1H); whereas in the mammary alveolar, dendritic cells are generally non-responsive to MPEC infection. However, when efferocytosis done by macrophages is ineffective, there is increased clearing of apoptotic neutrophils by dendritic cells. Curing of the mammary gland is only successful if minimal tissue damage occurs.

14Clinical mastitis is easily detected by visible abnormalities in the udder (e.g., redness, swelling, increased temperature, and pain upon physical palpation) and milk (e.g., presence of clots to clear, serum-like, or bloody secretions). In subclinical mastitis, however, indirect inflammation measures must be used to detect the disease. Contrarily to clinical mastitis, the only visible sign of animals with subclinical mastitis is reduced milk production. Consequently, the diagnostic requires complementary tests based on the SCC of milk. The SCC is used as an indicator of mammary gland health and milk quality. Usually, healthy cows produce milk with less than 100,000 SCC/mL, while cows with mastitis have a minimum SCC of 200,000 SCC/mL. The California Mastitis Test (CMT) estimates the SCC on milk and can be performed at the cow-side to detect mastitis. Electronic SCC is another test performed on the total mixed (composite) milk from all quarters of each cow, and it helps track the progress of control programs. Microbiologic culture of milk from individual quarters or composite samples of individual animals is another tool to identify E. coli involved in clinical mastitis. However, since the number of bacteria may be too small to be identified by standard methods, bacteria are not detected in roughly 30% of samples. Repeated culture and specialized culture techniques may be required in these cases.

15Because of the high costs of E. coli mastitis and the importance of maintaining cattle’s health and well-being, early diagnosis is critical. As previously mentioned in Section “Bovine Mastitis Diagnosis,” SCC and CMT are techniques commonly used to diagnose bovine mastitis. Unfortunately, both techniques have drawbacks, including subjective interpretation of CMT results, which could result in false positives and negatives, and factors such as lactation number, milk yield, stress, season, and breed that could influence with SCC’s interpretation. As a result, genotypic approaches are also used to supplement phenotypic identification of bovine mastitis, providing a viable option for resolving issues such as false-negative results. For example, approximately 30% of clinical mastitis samples do not grow in bacterial culture; however, polymerase chain reaction (PCR) is sensitive to detect growth-inhibited and reduces false-negative results. Other molecular typing methods used for the identification of bovine mastitis pathogen at the species level include ribotyping and amplified fragment length polymorphism (AFLP); at the strain level, include restriction fragment length polymorphism (RFLP), pulsed-field gel electrophoresis (PFGE) typing, and multiple-locus variable-number tandem repeat analysis (MLVA); and at both species and strain levels include transfer DNA intergenic spacer length polymorphism and DNA sequencing of housekeeping genes. Another genotypic method is microarray technology, which can detect seven common species of mastitis-causing pathogens in as little as 6 h, with a sensitivity of 94.1% and a specificity of 100%.

16New biomarkers for high sensitivity and specificity, rapid and efficient with a “cow-side” application, are among the development prospects for novel E. coli mastitis diagnosis techniques. For example, proteomic research has yielded data on protein expression patterns that can be used to find novel therapeutic targets (e.g., bacterial immunogenic proteins for vaccines) and diagnostic biomarkers for bovine mastitis. Importantly, a detection method’s suitability for routine mastitis diagnosis is determined by its specificity, sensitivity, cost, processing time, and ability to handle a large number of milk samples. Proteomic technique for accurate biomarkers is feasible for early identification of mastitis and treatment efficacy, as well as the discovery of possibly novel targets for alternative therapy development. These advancements, however, are still not suitable for routine diagnosis of bovine mastitis.

17Since mastitis results from the interaction of many different factors, especially milking procedures and cleanliness of the environment, there is no single way to treat and control the disease. The control of environmental mastitis is especially challenging because E. coli is ubiquitous in the cattle’s environment. Thus, good herd health management and environmental hygiene are the most critical factors in preventing this type of mastitis. Except for severe infections during the puerperal period, antibiotic therapy for coliform mastitis is not recommended due to the high spontaneous cure rates and the risk of antibiotic contamination of the bulk milk. Supportive care such as fluid therapy and treatment with steroidal or non-steroidal anti-inflammatory drugs (e.g., dexamethasone) should be the first treatment option. Parenteral administration of fluoroquinolones (e.g., enrofloxacin, danofloxacin, marbofloxacin) or third-generation cephalosporin (e.g., ceftiofur) is recommended to treat severe E. coli mastitis.

18Another approach to prevent coliform mastitis is to boost the animal’s specific immunity through vaccination, particularly with the E. coli J5 vaccine. While the teat canal is the first line of defense against intramammary infections, milk neutrophils are the major immune defense when E. coli has already invaded the teat canal. When intramammary infections are not quickly controlled by neutrophils, allowing LPS and lipoproteins to escape into the bloodstream, antibodies in the blood serum must be present to neutralize these potent toxins. Thus, preventing coliform mastitis through vaccination aims to achieve high levels of effective anti-coliform antibodies and antitoxins, both in milk and in the bloodstream, and around the teat canal. Two mutant strains of bacteria, E. coli O111:B4 (J5) and Salmonella Typhimurium Re-17 have been used in commercial preparations to immunize cows against coliform mastitis. E. coli O111:B4 (J5) is a rough mutant strain with an incomplete “O” polysaccharide chain in the cell wall, exposing the homologous half of the LPS core antigen (Zaatout, 2021). When this antigen is exposed, it stimulates the synthesis of immunoglobulins (e.g., IgM, IgG1, IgG2) that cross-react with other bacteria’s core antigens, resulting in immunity and protection against a wide range of bacterial genera and strains. The J5 vaccination boosts blood and milk titers of specific antibodies against E. coli LPS, enhancing its opsonization. Importantly, neutrophils can phagocyte bacteria more effectively when pathogens are opsonized by antibodies. Vaccinations are often timed to correspond with the highest risk of acquiring mastitis by coliforms (e.g., pre- and immediate postpartum), with immunizations being carried out 30 days before delivery and in the first week following delivery. Increasing the cattle’s natural ability to fight infections in the mammary gland during periods of increased susceptibility is one alternative for reducing mastitis in dairy cows.

19The outer membrane protein A (OmpA) has also been considered a potential antigen in the bovine mastitis vaccine associated with E. coli. OmpA is a crucial virulence factor in the pathogenesis of E. coli, playing a key role in pore and biofilm formation, host cell invasion, and multidrug resistance. For example, recombinant OmpA protein fragments could regulate the expression of cytokines, chemokines, nitric oxide synthase, and cyclooxygenase-2, preventing E. coli meningitis in mice. Moreover, OmpA has been found to reduce complement-dependent or phagocytic-dependent killing of OmpA-positive E. coli. Therefore, OmpA is a potential antigen in vaccine development and for the prevention of E. coli infection. However, despite current investigations on mastitis vaccine, there is limited information on OmpA as a viable vaccine antigen in mastitis-associated E. coli. Among the few studies on OmpA as a vaccine candidate in E. coli mastitis, investigated the immunogenicity of OmpA by immunizing cows with a recombinant protein (rEcOmpA) to potentially stimulate antibodies and cell-mediated immune responses. Interestingly, the intramammary immunization elicited antibodies (e.g., IgG1 and IgG2), but the antibodies could not interact with the mastitis-causing strain P4, implying that they were unable to reach the accessible regions of OmpA molecules located at the outside leaflet of the bacteria external membrane. Collectively, the aforementioned studies show that the potential of OmpA as a vaccine to prevent E. coli-associated bovine mastitis has yet to be determined.

20Antibiotics are crucial in curing serious cases of bovine mastitis caused by E. coli. Several antimicrobial agents have been approved in the United States to treat E. coli mastitis, such as tetracyclines, macrolides, sulphonamides, quinolones, and β-lactams. Unfortunately, antibiotic therapy has frequently been demonstrated to have little or no effect in treating clinical or subclinical mastitis caused by coliforms in cattle. This fact is evidenced by the increase in antimicrobial-resistant isolates and the growing number of recurrent and persistent mastitis cases, often caused by the same E. coli isolate. E. coli isolated from the milk of mastitis-affected cattle is resistant to numerous antibiotic classes, including but not limited to aminopenicillin (e.g., cloxacillin), polypeptide (e.g., bacitracin), lincosamide (e.g., lincomycin), and macrolide (e.g., erythromycin). In clinical mastitis, additional caution is advised when determining whether or not to treat cattle with antimicrobials, as antibiotics are not always essential, such as in cases of negative cultures and minor coliform infections. However, severe cases of mastitis caused by E. coli must be treated immediately following diagnosis due to endotoxin-induced shock.

21Pathologic alterations in the mammary gland, such as necrosis and ischemia, might obstruct antimicrobial distribution in the udder parenchyma, preventing the antimicrobial from reaching the site of infection or reaching sub-inhibitory concentrations. Importantly, the ability of E. coli to form biofilms on the tissue surface of the infected mammary gland, thereby establishing an inherent resistance to several antimicrobials treatment, is one of the hypotheses to explain the emergence of recurring mastitis. E. coli is notorious for being a reservoir for antimicrobial resistance genes, and it has the ability to horizontally transmit such genes to other pathogenic bacteria. Investigations have shown that the most prevalent resistance genes found in mastitis-associated E. coli encode resistance to aminoglycoside (e.g., aadA), streptomycin (e.g., strA, strB), tetracycline (e.g., tetA, tetC), sulfonamide (e.g., sulI, sulII), ampicillin (e.g., ampC), and β-lactams (e.g., blaTEM, blaCTX-M). Importantly, the extended-spectrum β-lactamase enzymes (ESBL) are a major concern in the antibiotic resistance mechanism of E. coli mastitis. These enzymes are located in plasmids and can hydrolyze penicillin, third and fourth-generation cephalosporins, and monobactam antibiotics. The detection of ESBL-producing isolates is of concern due to the direct association between multi-resistant bacteria and high mortality rates in humans. There is a clear increase in the prevalence of ESBL E. coli intestinal carriage among healthy individuals worldwide (2.6% in 2003–2005 to 21.1% in 2015–2018; an average increase of 1.2% per year). Importantly, several countries, including China, Egypt, Germany, and Greece, have been reporting the occurrence of ESBL-producing E. coli in cattle with mastitis. Moreover, the presence of ESBL-producing E. coli in milk raises concerns about the risks of ingesting unpasteurized milk and dairy products, potentially transmitting ESBL-producing E. coli to humans. The prevalence of these isolates in milk has ranged from 0.4% to 25.4%.

22Tests used to verify the antimicrobial sensitivity of pathogens causing mastitis, including antibiogram, disc diffusion technique, and minimum inhibitory concentration (MIC), are alternatives for the rational use of antimicrobials, aiming to support the treatment only of quarters infected by agents with the possibility of cure. Nowadays, laboratories’ most often used method is the MIC, which produces more accurate findings than the antibiogram. This technique, however, does not yield reliable results when assessing sessile cells since sensitivity studies on bacteria organized in biofilms reveal that the concentration of antibiotics necessary for biofilm eradication is several times larger than that determined by MIC.

23Under field conditions, colonization of the mammary gland is a polymicrobial event. Since milk is an excellent growth medium, many species of bacteria grow on it, colonize the mammary gland, and elicit inflammation and disease. Nevertheless, most mastitis cases are attributed to a single bacterial species or even to a single strain or clone of a species. The ability of E. coli to acquire exogenous DNA, including virulence genes, contributes to the development of pathogenic strains adapted to the mammary gland and potentially cause mastitis. MPEC has been proposed as a new pathotype responsible for causing mastitis in dairy animals. Some mastitis-associated E. coli strains evolved mechanisms that enable them to compete with other bacteria co-infecting the gland and evading the protective mechanisms of the host. However, it is important to note that although the aforementioned virulence-associated properties benefit E. coli to thrive in the mammary gland environment, they are not unique to mastitis-associated E. coli, but are also common in ExPEC. It is crucial to understand the principles behind the infection strategy of E. coli so that new approaches to treat and prevent mastitis by this bacteria and perhaps other mammary pathogens can be developed. The use of antibiotics in farm animals and specifically to treat mastitis is gradually precluded, and thus new treatment and prevention strategies are needed.

24DG and MM wrote the manuscript. DG developed figure and table. MM revised manuscript. Both authors contributed to the article and approved the submitted version.

25The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

26All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.