Biofilm Formation by Multidrug-Resistant Serotypes of Salmonella Isolated from Fresh Products: Effects of Nutritional and Environmental Conditions

Abstract

Salmonella serotypes can develop biofilms in fresh food products. This study focused on determining the antimicrobial resistance profile and the effects of different growth media and environmental conditions on biofilm formation by multidrug-resistant serotypes of Salmonella. All 49.4% of the Salmonella strains (five serotypes) were multidrug resistant. Assessment of the ability to form biofilms using the crystal violet staining method revealed that 95.6% of the strains of Salmonella were strong biofilm producers in 96-well polystyrene microtiter plates. Overall, 59.3% of the Salmonella strains showed the rdar (red dry and rough colony) morphotype, 2.1% pdar (pink dry and rough colony), 27.4% bdar (brown dry and rough colony) and 10.9% saw (smooth and white colony), at two temperatures (22 and 35 °C). Mono-species biofilms of Salmonella serotypes showed a mean cell density of 8.78 log10 CFU/cm2 ± 0.053 in TSBS (1/20 diluted TSB (tryptic soy broth) + 1% strawberry residues) and 8.43 log10 CFU/cm2 ± 0.050 in TSBA (1/20 diluted TSB + 1% avocado residues) on polypropylene type B (PP) (p < 0.05). In addition, epifluorescence microscopy and scanning electron microscopy (SEM) enabled visualizing the bacteria and extracellular polymeric substances of biofilms on PP. Salmonella form biofilms depending on the serotype of the strains and the environmental conditions. Mono-species biofilms formed by Salmonella serotypes respond to nutrient limitation with the use of simplified culture media such as TSBA and TSBS.