Characterisation and antimicrobial activity of Pediococci spp. isolated from South African cheese

Abstract

Pediococci are Gram-positive, non-spore forming lactic acid bacteria, LAB, that are commonly used in the industrial fermentation of vegetables and meat. On the other hand species of pediococci may constitute part of the secondary microflora as non-starter lactic acid bacteria, NSLAB, responsible for cheese ripening. This study was conducted to determine and isolate pediococci from South African farm-style cheese and to morphologically and physiologically characterize the pediococci isolates to a species level. Strains of Pediococcus species isolated from South African farm-style cheese were evaluated for the production of antimicrobial peptides, pediocins, and their effect against food pathogens Bacillus cereus ATCC 1178 and Listeria monocytogenes ATCC 7644. Eight farm-style cheeses namely pasteurized young Gouda (PYG), pasteurized matured Goude (PMG) as well as pasteurized matured Parmesan (PMP); un-pasteurised or raw milk aged Bouquet (RAB), aged Gouda (RAG) and matured Gouda (RMG) as well as goat cheese Gouda (:RGG) were chosen for the isolation of Pediococcus species. LAB were cultivated on MRS agar where logarithmic counts of LAB ranged from 6.90 cfu/g to 9.40 cfu/g. Microscopic examination of selected colonies identified 110 (18%) of 606 isolates as Gram-positive, catalase negative presumptive pediococci occurring in pairs, clusters and tetrads. Presumptive pediococci were distributed among five of the eight cheeses namely PYG, PMG, RAB, RAG and RMG in numbers of 33, 21,28, 12 and 16 respectively. Physiological characterization of presumptive pediococci isolates was determined under specific growth parameters, temperature, pH and salt (w/v). These isolates were characterized as P. acidilactici, forty-nine, and P. pentosaceus, sixty-one, isolates. For the antimicrobial assay three techniques namely, the agar disc, spot and overlay methods were evaluated. Comparisons on the sensitivity or susceptibility of two Lactococcus strains, L. diacetilactis NCDO 176 and L. lactis NCDO 605, to crude extract from P. acidilactici STI was assessed. The agar disc assay technique produced more reliable results compared to the other techniques and L. diacetilactis NCDO 176 was more susceptible to crude pediocin extract produced from P. acidilactici STI. A total of fifty-two strains (47%) from both species, twenty-seven (24%) P. acidilactici and twenty-five (23%) P. pentosaceus, exerted antagonism against L. diacetilactis NCDO 176 through the action of perdiocins. Among these strains thirteen (13%) of both species, seven (6%) P. acidilactici and six (7%) P. pentosaceus inhibited B. cereus ATCC 1178 while a total of thirty-seven strains (33%) of both species where, seventeen (15%) P. acidilactici and twenty (18%) P. pentosaceus showed inhibition against L. monocytogenes ATCC 7644. Among these strains inhibition of food pathogens was variable against L. monocytogenes ATCC 7644 and low against B. cereus ATCC 1178. Comparison of the antimicrobial activity of the two Pediococcus species showed similarity in the inhibition pattern, however, more strains of P. pentosaceus exerted antagonism against L. lactis NCDO 176 and L. monocytogenes ATCC 7644 compared to P. acidilactici.