Inhibition of Foodborne Pathogenic Bacteria by Excreted Metabolites of Serratia marcescens Strains Isolated from a Dairy-Producing Environment

Abstract

Serratia marcescens strains from a dairy-producing environment were tested for their inhibitory effect on Listeria monocytogenes, Salmonella Hartford, Yersinia enterocolitica and Escherichia coli. Inhibition of foodborne pathogens was observed in the case of a non-pigmented Serratia strain, while the pigment-producing isolate was able to inhibit only Y. enterocolitica. The co-culturing study in tryptone soya broth (TSB) and milk showed that the growth of Salmonella was inhibited in the first 24 h, but later the pathogen could grow in the presence of the Serratia strain even if its cell concentration was 1000 times higher than that of Salmonella. However, we found that (1) concentrated cell-free supernatants had stronger inhibitory activity, which confirms the extracellular nature of the antagonistic compound(s). We proved that (2) protease and chitinase enzymes can take part in this mechanism, but they are not the main inhibitory compounds. The presence of prodigiosin was observed only in the case of the pigmented strain; thus, (3) we hypothesized that prodigiosin does not take part in the inhibition of the pathogens. However, (4) the combined effect of different extracellular metabolites might be attributed to the inhibitory property. Application of concentrated S. marcescens cell-free supernatant can be an effective antibacterial strategy in the food industry, mainly in the form of a bio-disinfectant on surfaces of food-processing areas.

Keywords: antibacterial activity; extracellular metabolites; food safety; hydrolytic enzymes; natural antimicrobials; prodigiosin.

Figure 1

Effect of S. marcescens CSM-RMT-1 and CSM-RMT-II-1 strains on growth of Y. enterocolitica, L. monocytogenes and E. coli after 6 days of incubation at 30 °C detected with the agar spot method. In the case of Salmonella Hartford the zone was detected at 10 °C on day 3.

Figure 2

(A–D). Effect of non-lyophilised and concentrated (10×) cell-free supernatants of S. marcescens CSM-RMT-1 on growth of the tested foodborne pathogenic bacteria ((A): Salmonella Hartford, (B): E. coli, (C): L. monocytogenes, (D): Y. enterocolitica).

Figure 2

(A–D). Effect of non-lyophilised and concentrated (10×) cell-free supernatants of S. marcescens CSM-RMT-1 on growth of the tested foodborne pathogenic bacteria ((A): Salmonella Hartford, (B): E. coli, (C): L. monocytogenes, (D): Y. enterocolitica).

Figure 3

Proteolytic activities of S. marcescens CSM-RMT-1 and CSM-RMT-II-1 strains after 24, 48 and 120 h of incubation at three different (20, 25 and 30 °C) temperatures. Sizes of clearing zones are mean values of three parallel measurements. The bars indicate the standard deviation of three replicates of the experiment. Different upper-case letters between the incubation times indicate a significant difference between the clearing zone within the same strain according to Games–Howell post hoc tests (α = 0.05).

Figure 4

Co-culturing results of prodigiosin-negative S. marcescens CSM-RMT-1 and Sa. enterica in (A) TS broth and (B) 2.8% fat-containing UHT milk after 0, 1, 2, 3 and 6 days of incubation. Different upper-case letters by CLD indicate a significant difference between the cell numbers according to Tukey’s HSD post hoc tests (α = 0.05).