Purpose: This study compared the lactic acid resistance of individual strains of six nSTEC serotypes (O26, O45, O103, O111, O121 and O145; wild-type and spontaneous rifampicin-resistant variants), and antibiotic susceptible and multidrug resistant (MDR and/or MDR-AmpC) S. Newport and S. Typhimurium, to that of a 5-strain mixture of E. coli O157:H7.
Methods: After inoculation (6 log CFU/ml) of a sterile 10% (w/w) beef homogenate, lactic acid was added to a target concentration of 5%. At 0, 2, 4, 6 and 8 min, aliquots were analyzed (two repetitions, three acid challenges per strain/mixture each) for survivors. Data were analyzed as a randomized complete block design.
Results: Results showed that in most cases, individual strains of wild-type and rifampicin-resistant variants of nSTEC, and antibiotic susceptible and resistant S. Newport and S. Typhimurium were less (P < 0.05) acid tolerant than the E. coli O157:H7 mixture of strains (wild-type and rifampicin-resistant). Inoculated wild-type and rifampicin-resistant E. coli O157:H7 strain mixtures (6.1 and 5.9 log CFU/ml, respectively) were reduced (P < 0.05) to 1.1 log CFU/ml and below the detection limit (<1.0 log CFU/ml) within 8 and 6 min, respectively. Out of the 35 nSTEC strains tested, 85.7% of the wild-type and 82.9% of the rifampicin-resistant variants reached the detection limit immediately after addition of lactic acid (0 min) or within 6 min of exposure. Similarly, 87.9% of the 33 S.Newport/Typhimurium strains reached the detection limit within 0 to 4 min, irrespective of serovar or antibiotic resistance phenotype.
Significance: The majority of the tested individual strains of nSTEC and the antibiotic susceptible and resistant S. Newport/Typhimurium showed lower acid resistance compared to E. coli O157:H7 indicating that these pathogens can be eliminated and/or reduced by lactic acid interventions at least as effectively as E. coli O157:H7 during beef slaughter operations.