SURVIVAL OF ACID AND SALT STRESSED LISTERIA MONOCYTOGENES IN CHEESE.

Discipline: contamination; Key words: Listeria monocytogenes, listeriosis, soft cheese, stress

Listeria monocytogenes causes the disease listeriosis. The organism is often present in food processing factories, including cheese manufacturing plants and therefore can penetrate the manufacturing process. Among dairy products soft cheeses are particularly vulnerable and therefore are leading products causing listeriosis in many parts of the world. Although sensitive to stress variables such as acid and salt conditions, continuous exposure to these variables may result in selection pressure for resistant variants with enhanced survival. This was the topic of the study of the authors cited below.

In their study, L. monocytogenes strains were subjected to acid and salt (NaCl) stress which were inoculated into soft cheese and stored at 4⁰C for 15 days. The survival of the strains was determined using appropriate descriptive models, whereas PCR fingerprinting was used to assess the diversity of the survivors after 15 days.

The descriptive models proved satisfactory to determine survival rate. The results showed that both acid and salt stressed L. monocytogenes cells had significantly reduced inactivation rates. Exposure to stress likely results in the death of susceptible cells leaving only the harder less heterogenous cells that are more resistant to cheese acid and salt additives. The further addition of sodium lactate and diacetate resulted in an increase in the inactivation time. Of the 40 isolates which survived the stress variables, strain 159/10 was prominent after 15 days of exposure.

The results proved that when L. monocytogenes was exposed to the stress variables used, it will enhance its survival in soft cheese. The response, however, depends on the presence of others as individual strains survive differently when in mixed strains.

Reference:

Thulani Sibanda & Elna Buys, 2017. Modelling the survival of acid and salt stressed Listeria monocytogenes in a lactic soft cheese stored at 4⁰C. In: Proc. of the 50^th SASDT Symp. “Dare to Dairy”, Kievits Kroon, Pretoria, 10-11 May 2017.