Antibacterial activity of carvacrol and 2-nitro-1-propanol against single and mixed populations of foodborne pathogenic bacteria in corn flour dough

Cereal doughs are an important part of human diet, but at the same time can act as vehicles for the transmission of human pathogenic bacteria. In the present study, four pathogenic or toxinogenic bacteria (Escherichia coli O157:H7, Salmonella enterica serovar Enteritidis, Bacillus cereus and Staphylococcus aureus) were inoculated in a dough made from corn flour in combination with the single antimicrobial compounds carvacrol and 2-Nitro-1-propanol (2NPOH). Survival of single and mixed populations in the treated doughs incubated at 37 °C was followed by culture-dependent and independent methods (TTGE). All strains were completely inactivated within 24 h by the tested compounds at 5% final concentration, but showed variable inhibition at lower concentrations of 0.5% and 2%. Sensitivity to antimicrobial compounds was in general modified when strains were tested in cocultures compared with single cultures. B. cereus was more sensitive to carvacrol (minimum bactericidal concentration, MBC, 0.5%) in coculture with S. aureus. It was also more sensitive to 2NPOH in cocultures with S. aureus and with S. enterica (MBC, 2% in both cases). S. aureus was more resistant to carvacrol (MBC, 5%) in cocultures with B. cereus, E. coli, as well as S. enterica. However, sensitivity to 2NPOH was not modified in any of the coculture experiments (MBC, 2%). E. coli was more resistant to carvacrol (MBC, 5%) in cocultures with S. aureus and with S. enterica. Resistance of E. coli to 2NPOH also increased in cocultures with B. cereus (MBC, 5%) and with S. aureus (MBC, 2%), but not with S. enterica (MBC, 0.5%). S. enterica was more resistant to carvacrol (MBC, 5%) in cocultures with E. coli, but it was more sensitive to 2NPOH in cocultures with B. cereus as well as with S. aureus (MBC, 2% in both cases). TTGE analysis of survivors from cocultures treated with 2NPOH or carvacrol allowed a good estimation of the identity of survivors according to their DNA band patterns. Results from this study indicate that the efficacy of antimicrobials such as carvacrol and 2NPOH is greatly influenced by the complexity of the microbial populations under target and the relationships between individual populations.