Development and PFGE monitoring of dominance among spoilage lactic acid bacteria from cured meats

Abstract

Strains of lactic acid bacteria (LAB), isolated from intact vacuum-packaged cured meats obtained at retail, were subjected to single inhibitory factor and mixed-culture dominance tests during challenge with different conditions of pH and temperature in the presence of NaCl and NaNO2to study reasons for strain dominance. Bacteria were checked for bacteriocin production. Both initial pH and temperature had a significant effect on the growth of spoilage LAB isolates in modified MRS broth. Lactobacilli generally grew better than Lc. mesenteroides at pH 5·5. It appeared that L. curvatus and L. sakei grew faster at elevated temperature (12°C) than Lc. mesenteroides. It was also noted that there was a similar response for strains within the same species to the challenges of NaCl, NaNO2, pH and temperature. When present in mixed cultures, Leuconostoc strains did not grow well at 2°C and at an initial pH of 5·5 compared with lactobacilli, but performed better at this low temperature as pure cultures than when mixed with lactobacilli. At normal cured meat pH (6·0 and 6·5) and higher temperatures (6°C and 12°C), dominant bacteria always grew from the originally larger bacterial population. When leuconostocs and lactobacilli were present in equal initial numbers, leuconostocs generally did not grow as well as lactobacilli. Lc. mesenteroides grew faster than L. curvatus at 6°C but did not out-compete L. sakei at this temperature (pH ≥ 6·0). Pulsed field gel electrophoresis (PFGE) of SmaI digested genomic DNA successfully distinguished all the strains under study. Difficulties associated with enumeration of LAB having similar biochemical properties were addressed by the development of a composite-simultaneous PFGE method for interspecies qualification using single colonies from agar plates to generate digested DNA for analysis.