Biofilm-Forming Abilities of Listeria monocytogenes Serotypes Isolated from Different Sources.

Swapnil P Doijad,Dewanand R Kalorey,Krupali V Poharkar,Sandeep Garg,Nitin V Kurkure,Sukhadeo B Barbuddhe,Trinad Chakraborty,Deepak B Rawool,Niyaz Ahmed

doi:10.1371/journal.pone.0137046

Abstract

A total of 98 previously characterized and serotyped L. monocytogenes strains, comprising 32 of 1/2a; 20 of 1/2b and 46 of 4b serotype, from clinical and food sources were studied for their capability to form a biofilm. The microtiter plate assay revealed 62 (63.26%) strains as weak, 27 (27.55%) strains as moderate, and 9 (9.18%) strains as strong biofilm formers. Among the strong biofilm formers, 6 strains were of serotype 1/2a and 3 strains were of serotype 1/2b. None of the strain from 4b serotype exhibited strong biofilm formation. No firm correlation (p = 0.015) was noticed between any serotype and respective biofilm formation ability. Electron microscopic studies showed that strong biofilm forming isolates could synthesize a biofilm within 24 h on surfaces important in food industries such as stainless steel, ceramic tiles, high-density polyethylene plastics, polyvinyl chloride pipes, and glass. Cell enumeration of strong, moderate, and weak biofilm was performed to determine if the number of cells correlated with the biofilm-forming capabilities of the isolates. Strong, moderate, and weak biofilm showed 570±127× 103 cells/cm2, 33±26× 103 cells/cm2, 5±3× 103 cells/cm2, respectively, indicating that the number of cells was directly proportional to the strength of the biofilm. The hydrophobicity index (HI) analysis revealed higher hydrophobicity with an increased biofilm formation. Fatty acid methyl esterase analysis revealed the amount of certain fatty acids such as iso-C15:0, anteiso-C15:0, and anteiso-C17:0 fatty acids correlated with the biofilm-forming capability of L. monocytogenes. This study showed that different strains of L. monocytogenes form biofilm of different intensities which did not completely correlate with their serotype; however, it correlated with the number of cells, hydrophobicity, and amount of certain fatty acids.

Highlights

Listeria monocytogenes is a gram-positive bacterium, emerging as a foodborne pathogen and the etiological agent of listeriosis
Average turbidities of growth and crystal violet stain of L. monocytogenes estimated after 24 h incubation at 37°C are shown in Fig 1a, 1b and 1c
Listeria monocytogenes poses a serious threat to public health, and the majority of cases of human listeriosis are associated with the consumption of contaminated food

Summary

Introduction

Listeria monocytogenes is a gram-positive bacterium, emerging as a foodborne pathogen and the etiological agent of listeriosis. For L. monocytogenes, such activities supported by factors such as growth at the wide pH range, salt tolerance, growth at low temperature, resistance to different stress conditions and biofilm formation ability. L. monocytogenes has been shown to occur on the surfaces in food industries that may directly or indirectly come in contact with the food leading to the contamination. Several such industrially processed foods, such as cheese, meat, have been reported to be contaminated with L. monocytogenes [12, 13]. Several researchers have tried to relate L. monocytogenes serotypes with their abilities to adhere, to form biofilm, to resist disinfectant or antibiotic, and to tolerate stress [18]; certain results remain contradictory or inconclusive [19,20,21]. The analysis of L. monocytogenes isolates from diverse sources having different genetic make ups is necessary to deduce a relation, if any, between biofilm formation and serotypes

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