Determination of antimicrobial susceptibility (AMS) of Escherichia coli causing clinical mastitis (CM) according to the phylogenetic groups and its association with descriptors at the cow and herd level may help improve specific strategies for treatment and control of this pathogen in dairy herds. The aims of the present study were to (a) determine the frequency of phylogenetic groups of E. coli isolated from CM in dairy cows, and its association with cow-level descriptors (parity, lactation stage, CM severity, and affected quarter position), housing system, and season; and (b) determine and compare AMS among E. coli phylogenetic groups. A quadruplex PCR method was used to classify E. coli isolates into 1 of the 7 phylogenetic groups. Minimal inhibitory concentrations were determined for 10 antimicrobials, and survival analysis was performed to evaluate the AMS differences among E. coli phylogroups. Most E. coli isolates belonged to phylogroups A (52%) and B1 (38%). None of the cow- and herd-level descriptors were associated with the E. coli phylogenetic groups. Overall, E. coli isolates were mostly susceptible to ceftiofur (96.8%), sulfadimethoxine (75.5%), and cephalothin (74.5%). Based on the survival analysis, differences in AMS between phylogenetic groups of E. coli was observed only for cephalothin, in which strains of phylogroup A were inhibited at lower minimum inhibitory concentration than strains of phylogroup B1. Results of this study indicated low susceptibility of E. coli isolates identified from CM to most antimicrobials. In addition, differences in AMS can occur among E. coli phylogenetic groups, although they may be uncommon as they were limited to only one antimicrobial (i.e., cephalothin).

根据系统发育群体确定引起临床乳腺炎（CM）的大肠杆菌的抗菌药敏性（AMS），并将其与奶牛和奶牛水平的描述因素联系起来，可能有助于改善奶牛群中这种病原体的治疗和控制策略。本研究的目的是（a）确定奶牛从CM中分离出的大肠杆菌系统发生群的频率，及其与奶牛水平指标的关联（胎次，泌乳阶段，CM严重程度和受影响的四分之一位置） ，住房制度和季节；（b）确定和比较大肠杆菌系统发生群中的AMS 。使用四重PCR方法对大肠杆菌进行分类分离为7个系统发育组中的1个。确定了10种抗菌剂的最低抑菌浓度，并进行了生存分析，以评估大肠杆菌菌群之间的AMS差异。大多数大肠杆菌分离物属于系统群A（52％）和B1（38％）。牛和畜群水平的描述子均与大肠杆菌的系统发生组无关。总体而言，大肠杆菌分离物最容易感染头孢噻呋（96.8％），磺胺二甲氧嘧啶（75.5％）和头孢菌素（74.5％）。基于生存分析，大肠杆菌系统发生群之间的AMS差异仅在头孢菌素中观察到了这种情况，其中头孢菌素组的菌群A菌株的最低抑菌浓度要低于头孢菌素B1的菌种。这项研究的结果表明，从CM中鉴定出的大肠杆菌分离株对大多数抗菌剂的敏感性较低。此外，在大肠杆菌系统发生组中，AMS的差异可能会发生，尽管它们可能并不常见，因为它们仅限于一种抗菌剂（即头孢菌素）。