Increased Salt- and Nisin-Sensitivity of Pressure-Injured BioluminescentListeria Monocytogenes

Abstract

Suspensions of a bioluminescent (luxAB) transformant of Listeria monocytogenes in pH 7.0 phosphate buffer were pressurised and the effect of the pressure treatment was monitored by plate counting. When the bacteria were suspended in NaCl- and nisin-free buffer the number of colony forming units (CFU) decreased by 3 and 6 log cycles after 300 MPA for 10 and 30 min, respectively. Supplementing the plating medium with 5% NaCl did not influence the colony forming capacity of non-pressurised cells, however, CFU of residual populations after respective treatments of 300 MPa for 10 and 30 min were reduced by a further 2 and 3.5 log cycles in case of salt containing plates. Nisin-addition to the plating medium caused less than one log unit decrease in the CFU of the non-pressurised population. However, the CFU of 10 min-pressurised sample was 4 log cycles less in the nisin-containing plates than in the nisin-free ones, whereas no colonies were formed in the nisin-containing plates even when 1 ml was inoculated from the originally 10(10) CFU/ml population after 300 MPa for 30 min. The luciferase activities (bioluminescence intensities) decreased concomitant with the reduction of the viable cell counts, however, they were approx. 0.6-0.8 log units less in the presence of 5% NaCl in the pressurised suspension than those expected from the previously established linear correlation between the logarithmic light outputs and the logarithmic viable cell counts.

Evaluation of ALOA plating medium for its suitability to recover high pressure-injured Listeria monocytogenes from ground chicken meat

“…1986; Smith and Archer 1988). Farkas et al. (2003) have previously shown that pressure‐injured L. monocytogenes showed increased salt sensitivity.…”

“…(2003) have previously shown that pressure‐injured L. monocytogenes showed increased salt sensitivity. They found that supplementing brain heart infusion agar plates with 5% NaCl did not influence the recovery of nonpressurized cells but residual populations of treated cells were reduced by several log 10 cycles in case of NaCl containing plates (Farkas et al. 2003).…”

“…Briefly, these methods are based on the observation that both uninjured and sublethally injured cells of L. monocytogenes can form colonies on nonselective plating media, but injured cells fail to produce colonies on plating media containing 4% NaCl (Beuchat et al 1986;Smith and Archer 1988). Farkas et al (2003) have previously shown that pressure-injured L. monocytogenes showed increased salt sensitivity. They found that supplementing brain heart infusion agar plates with 5% NaCl did not influence the recovery of nonpressurized cells but residual populations of treated cells were reduced by several log 10 cycles in case of NaCl containing plates (Farkas et al 2003).…”