The researchers, whose findings were published in the journal Food Microbiology, reported that it is important to develop combined treatments of the two antimicrobial compounds to increase the final bactericidal effect and reduce the concentrations used.
Cereal doughs can act as vehicles for the transmission of human pathogenic bacteria, claim the authors, and they point out that various production steps including packaging may contribute to food spoilage and the growth of undesirable food-poisoning bacteria so new food additives are necessary to help mitigate this risk.
The scientists, based at the University of Jaén in Spain, found that while all strains of four pathogenic or toxinogenic bacteria (Escherichia coli O157:H7, Salmonella enterica serovar Enteritidis, Bacillus cereus and Staphylococcus aureus) were completely inactivated within 24 hours by the tested compounds at a concentration of 5 per cent, they showed variable inhibition at lower concentrations of 0.5 per cent and 2 per cent.
And sensitivity to antimicrobial compounds was in general modified when strains were tested in cocultures compared with single cultures, they added.
The latest findings complement and extend other studies which also identified the inactivation of foodborne pathogens by the same antimicrobials, said the authors.
The researchers explained that corn flour-based doughs were inoculated with single pathogens using Escherichia coli O157:H7, Salmonella enterica serovar Enteritidis, Bacillus cereus and Staphylococcus aureus and with combinations of two of these pathogenic bacteria.
Control samples were contaminated with bacterial suspensions in the absence of added antimicrobials, they said.
The authors said that the antimicrobial effects of the compounds against the bacteria was investigated by using an approach combining microbial enumeration with selective culture media and temporal temperature gradient gel electrophoresis (TTGE) fingerprinting of total community DNA.
Carvacrol was found to be more active in reducing the concentration of foodborne bacteria when corn flour-based doughs were inoculated with single pathogens and treated with the phenolic compound, as it reduced the concentration of the four pathogenic microorganisms studied below detection limits at a concentration of 2 per cent after 24 hours incubation (its minimal bactericidal concentration, or MBC).
“At 0.5 per cent, carvacrol also caused a significant inhibition of growth for S. enterica and E. coli and decreased the survival for B. cereus,” said the authors.
They said that, among the bacteria tested, E. coli was the most sensitive to 2NPOH (MBC, 0.5 per cent) followed by S. aureus (MBC, 2 per cent). However, it was necessary to add a concentration of 5 per cent to obtain the same effects in the samples inoculated with S. enterica or B. cereus.
In the untreated controls, all the bacterial strains tested were able to multiply, reaching viable cell concentrations between 7.2 and 8.6 log CFU/g of food.
The researchers said that when corn flour-based doughs were simultaneously inoculated with combinations of two of the pathogenic bacteria and treated with antimicrobial compounds, the concentrations of bacteria were also significantly reduced but results were different on the basis of the combination inoculated.
And the researchers concluded that their results suggest that the efficacy of antimicrobial compounds in food systems should be tested on mixed bacterial populations rather than on single cultures.
Source: Food Microbiology
Published online ahead of print
Title: Antibacterial activity of carvacrol and 2-nitro-1-propanol against single and mixed populations of foodborne pathogenic bacteria in corn flour dough
Authors: E Ortega Morente, H Abriouel, R Lucas López, N Ben Omar, A Gálvez