Disinfectant and Antimicrobial Susceptibility Profiles of the Big Six Non-O157 Shiga Toxin–Producing Escherichia coli Strains from Food Animals and Humans

Abstract

The disinfectant and antimicrobial susceptibility profiles of 138 non-O157 Shiga toxin-producing Escherichia coli strains (STECs) from food animals and humans were determined. Antimicrobial resistance (AMR) was moderate (39.1% of strains) in response to 15 antimicrobial agents. Animal strains had a lower AMR prevalence (35.6%) than did human strains (43.9%) but a higher prevalence of the resistance profile GEN-KAN-TET. A decreasing prevalence of AMR was found among animal strains from serogroups O45 > O145 > O121 > O111 > O26 > O103 and among human strains from serogroups O145 > O103 > O26 > O111 > O121 > O45. One animal strain from serogroups O121 and O145 and one human strain from serogroup O26 had extensive drug resistance. A high prevalence of AMR in animal O45 and O121 strains and no resistance or a low prevalence of resistance in human strains from these serogroups suggests a source other than food animals for human exposure to these strains. Among the 24 disinfectants evaluated, all strains were susceptible to triclosan. Animal strains had a higher prevalence of resistance to chlorhexidine than did human strains. Both animal and human strains had a similar low prevalence of low-level benzalkonium chloride resistance, and animal and human strains had similar susceptibility profiles for most other disinfectants. Benzyldimethylammonium chlorides and C10AC were the primary active components in disinfectants DC&R and P-128, respectively, against non-O157 STECs. A disinfectant FS512 MIC ≥ 8 μg/ml was more prevalent among animal O121 strains (61.5%) than among human O121 strains (25%), which may also suggest a source of human exposure to STEC O121 other than food animals. Bacterial inhibition was not dependent solely on pH but was correlated with the presence of dissociated organic acid species and some undissociated acids.

Interactions of Organic Acids with Salmonella Strains from Feedlot Water-Sprinkled Cattle

“…If pH were the primary factor in inhibition of the bacteria, then one would expect the MICs for these same bacteria for all the different acids would be at the same pH value, but that is not the case. The Salmonella strains demonstrated similar behavior against the organic acids as did E. coli O157:H7 [34], non-O157 STECs [37] and P. aeruginosa [35], except the pH values for the E. coli O157:H7 strains were somewhat lower, possibly because O157:H7 strains are known to have multiple acid-resistance systems to protect them from extreme acid stress [42]. After comparing the study here of Salmonella strains to the previous three studies, E. coli O157:H7 [34], the non-O157 STECs [37] and P. aeruginosa [35], we observed a trend beginning to develop.…”

“…The Salmonella strains demonstrated similar behavior against the organic acids as did E. coli O157:H7 [34], non-O157 STECs [37] and P. aeruginosa [35], except the pH values for the E. coli O157:H7 strains were somewhat lower, possibly because O157:H7 strains are known to have multiple acid-resistance systems to protect them from extreme acid stress [42]. After comparing the study here of Salmonella strains to the previous three studies, E. coli O157:H7 [34], the non-O157 STECs [37] and P. aeruginosa [35], we observed a trend beginning to develop. The MIC M s of 100% of the Salmonella strains with acetic acid was at pH 4.9, 98.6% of non-O157 STECs with acetic acid was at pH 4.79 [37] and the MIC M s of 99% of the P. aeruginosa strains with acetic acid was at pH 4.9 [35].…”

“…After comparing the study here of Salmonella strains to the previous three studies, E. coli O157:H7 [34], the non-O157 STECs [37] and P. aeruginosa [35], we observed a trend beginning to develop. The MIC M s of 100% of the Salmonella strains with acetic acid was at pH 4.9, 98.6% of non-O157 STECs with acetic acid was at pH 4.79 [37] and the MIC M s of 99% of the P. aeruginosa strains with acetic acid was at pH 4.9 [35]. Since E. coli O157:H7 has glutamateand arginine-dependent acid-resistance systems to protect it from extreme acid stress [42], we see more acid-tolerance in E. coli O157:H7.…”

“…The disintegration of the LPS layer can be caused by a fully dissociable acid [36]. In a recent study of the big six non-O157 Shiga toxin-producing E. coli (non-O157 STEC) [37], overall, the dissociated acid concentrations were more closely correlated with the molar minimum inhibitory concentrations (MIC M s). However, both the dissociated acid form and the undissociated acid form may play a part in bacterial inhibition [36].…”

“…MICs were determined as the lowest concentration of a compound that showed no visible growth of the organism [40]. The organic acid susceptibility studies were carried out similarly to earlier studies with other pathogens [34,35,37]. Briefly, 50 µL of each organic acid solution was diluted 1:2 across a 96-well U-bottom Greiner bio-one microplate ((#82050-626, VWR, Houston, TX, USA) through column 11, and column 12 was the positive control.…”