Factors influencing the growth of Salmonella

Salmonella are rod shaped motile bacteria. They are gram-negative, non-spore-forming and facultative anaerobic. The genus salmonella has two species, each containing multiple serotypes. They are S. enterica and S. bongori.

Salmonella entrica is commonly associated with foodborne infections causing a spectrum of diseases ranging from self-limiting gastroenteritis to the life-threatening systemic disease, typhoid fever. The outcome of a disease mainly depends on the serotype of S. enterica.

The influence of external factors on the growth of Salmonella

The external factors affecting the rate of growth of Salmonella are; temperature, pH, salinity, water activity and available nutrients.

Temperature: They usually grow between 35-37ºC, though most Salmonella serotypes can grow at temperatures ranging from 5ºC to 47ºC although growth is reduced at temperatures below 10ºC. The minimum temperature of Salmonella is cardinal to the food industry as refrigeration constitutes one of the primary lines of defense against proliferation of this pathogen in human food.

Salinity: Salt affects the texture of meat products which in turn affect the growth and survival of microbial flora. Therefore there is a need to reduce salt intake by humans.

A comparison between the growth of Salmonella and Listeria monocytogenes at low temperature and PH

While processing food, microorganisms are usually exposed to a number of lethal or sub-lethal stresses sequentially or simultaneously. A study was carried out to evaluate the survival of Listeria monocytogens and Salmonella Typhimurium in lethal acid (pH 4.0 and 4.5 with lactic acid) or osmotic conditions (15-20% NaCl), applied singly, sequentially i.e. pH then then NaCl or NaCl then pH) or simultaneously at 5 and 10ºC. A second study was also carried out to quantify the effect of osmotic shifts at pH 7.0, 6.0 or 5.0 on the lag phase and growth rate of L. monocytogenes at 10ºC. The second stress was applied in a sequence of 2 or 3 days after the first for Salmonella and L. monocytogenes respectively. Acid tolerance in L. monocytogenes was higher than osmotic tolerance and vice versa for Salmonella. More inactivation was observed after exposure to pH 4.0 than pH 4.5 as well as after being exposed to 20% NaCl compared to 15% NaCl. Exposure to stresses sequentially caused faster reductions than the exposure to single or double stresses applied simultaneously. The pH then NaCl sequence proved to be more detrimental for pathogens than the reverse sequence. The variance in incubation temperature did not show any profound effect on microbial inactivation.

When L. monocytogenes was incubated at aw 0.930 and 0.995 at 30ºC, the lag phase increased in both subsequent osmotic downshift and upshift respectively at 10ºC. It was observed that there were shorter lag phase and higher ability to initiate growth at lower aw after pre-adaptation on pH 6.0 or 5.0 compared to pH 7.0.

These results may contribute to the review of critical limits to products characterized in low pH and water activity, considering the risk of L. monocytogenes and Salmonella survival. Similarly, these indications may address the need to apply stricter sanitation procedures in processing in order to minimize the risk of survivors.

Food safety Implications

Food safety relies mainly on the use of multiple hurdle technology. Depending on the processing, pathogens may be exposed to various stresses simultaneously like on ripened cheese or meat, or sequentially like, during fermentation and ripening of meat or milk. As shown in the study above, acid and osmotic stresses are the most common hurdles applied in food processing i.e. by acidification or adding NaCl, or naturally encountered in foods.

The sequence of hurdles has proven to be crucial for the survival of pathogens as the study has shown.

References:

Adams, M.R., Nicolaides, L., 1997. Review of the sensitivity of different foodborne pathogens to fermentation. Food Control 8, 227–239.

Bovill, R.A., Bew, J., Baranyi, J., 2001. Measurements and predictions of growth for Listeria monocytogenes and Salmonella during fluctuating temperature II. Rapidly changing temperatures. International Journal of Food Microbiology 67, 131–137.