Kay Kathryn L, Breidt Frederick, Fratamico Pina M, Baranzoni Gian M, Kim Gwang-Hee, Grunden Amy M, Oh Deog-Hwan
Department of Plant and Microbial Biology, North Carolina State UniversityRaleigh, NC, United States.
Food Science Research Unit, United States Department of Agriculture-Agriculture Research ServiceRaleigh, NC, United States.
Front Microbiol. 2017 Jul 26;8:1404. doi: 10.3389/fmicb.2017.01404. eCollection 2017.
Shiga toxin producing (STEC) strains vary in acid resistance; however, little is known about the underlying mechanisms that result in strain specific differences. Among 25 STEC O157:H7 strains tested, 7 strains flocculated when grown statically for 18 h in minimal salts medium at 37°C, while 18 strains did not. Interestingly, the flocculation phenotype (cells came out of suspension) was found to correlate with degree of acid sensitivity in an assay with 400 mM acetic acid solution at pH 3.3 targeting acidified foods. Strains exhibiting flocculation were more acid sensitive and were designated FAS, for flocculation acid sensitive, while the acid resistant strain designated PAR for planktonic acid resistant. Flocculation was not observed for any strains during growth in complex medium (Luria Bertani broth). STEC strains B201 and B241 were chosen as representative FAS (2.4 log reduction) and PAR (0.15 log reduction) strains, respectively, due to differences in acid resistance and flocculation phenotype. Results from electron microscopy showed evidence of fimbriae production in B201, whereas fimbriae were not observed in B241.Curli fimbriae production was identified through plating on Congo red differential medium, and all FAS strains showed curli fimbriae production. Surprisingly, 5 PAR strains also had evidence of curli production. Transcriptomic and targeted gene expression data for B201 and B241indicated that and (curli and acid induced chaperone genes, respectively) expression positively correlated with the phenotypic differences observed for these strains. These data suggest that FAS strains grown in minimal medium express curli, resulting in a flocculation phenotype. This may be regulated by GcvB, which positively regulates curli fimbriae production and represses acid chaperone proteins. RpoS and other regulatory mechanisms may impact curli fimbriae production, as well. These findings may help elucidate mechanisms underlying differences among STEC strains in relating acid resistance and biofilm formation.
产志贺毒素大肠杆菌（STEC）菌株的耐酸性存在差异；然而，对于导致菌株特异性差异的潜在机制知之甚少。在测试的25株STEC O157:H7菌株中，7株在37°C的最低盐培养基中静态培养18小时后出现絮凝，而18株没有。有趣的是，在针对酸化食品的pH 3.3的400 mM乙酸溶液测定中，絮凝表型（细胞从悬浮液中析出）与酸敏感性程度相关。表现出絮凝的菌株对酸更敏感，被指定为FAS（絮凝酸敏感型），而耐酸菌株被指定为PAR（浮游耐酸型）。在复杂培养基（Luria Bertani肉汤）生长期间，未观察到任何菌株出现絮凝。由于耐酸性和絮凝表型的差异，STEC菌株B201和B241分别被选为代表性的FAS（2.4个对数减少）和PAR（0.15个对数减少）菌株。电子显微镜结果显示B201有菌毛产生的证据，而在B241中未观察到菌毛。通过在刚果红鉴别培养基上平板培养鉴定卷曲菌毛的产生，所有FAS菌株均显示有卷曲菌毛产生。令人惊讶的是，5株PAR菌株也有卷曲菌毛产生的证据。B201和B241的转录组和靶向基因表达数据表明，csgD和gadA（分别为卷曲菌毛和酸诱导伴侣基因）的表达与这些菌株观察到的表型差异呈正相关。这些数据表明，在基本培养基中生长的FAS菌株表达卷曲菌毛，导致絮凝表型。这可能受GcvB调节，GcvB正向调节卷曲菌毛的产生并抑制酸伴侣蛋白。RpoS和其他调节机制也可能影响卷曲菌毛的产生。这些发现可能有助于阐明STEC菌株在耐酸性和生物膜形成方面差异的潜在机制。
