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.

Listeria monocytogenes is an important pathogen that has been implicated in foodborne illnesses and the recall of products such as fruit and vegetables. This study determines the prevalence of virulence-associated genes and serogroups and evaluates the effects of different growth media and environmental conditions on biofilm formation by L. monocytogenes. Eighteen L. monocytogenes isolates from Hass avocados sold at markets in Guadalajara, Mexico, were characterized by virulence-associated genes and serogroup detection with PCR. All isolates harbored 88.8% actA, 88.8% plcA, 83.3% mpl, 77.7% inlB, 77.7% hly, 66.6% prfA, 55.5% plcB, and 33.3% inlA. The results showed that 38.8% of isolates harbored virulence genes belonging to Listeria pathogenicity island 1 (LIPI-1). PCR revealed that the most prevalent serogroup was serogroup III (1/2b, 3b, and 7 (n = 18, 66.65%)), followed by serogroup IV (4b, 4d–4e (n = 5, 27.7%)) and serogroup I (1/2a–3a (n = 1, 5.5%)). The assessment of the ability to develop biofilms using a crystal violet staining method revealed that L. monocytogenes responded to supplement medium TSBA, 1/10 diluted TSBA, and TSB in comparison with 1/10 diluted TSB (p < 0.05) on polystyrene at 240 h (p < 0.05). In particular, the biofilm formation by L. monocytogenes (7.78 ± 0.03–8.82 ± 0.03 log10 CFU/cm2) was significantly different in terms of TSBA on polypropylene type B (PP) (p < 0.05). In addition, visualization by epifluorescence microscopy, scanning electron microscopy (SEM), and treatment (DNase I and proteinase K) revealed the metabolically active cells and extracellular polymeric substances of biofilms on PP. L. monocytogenes has the ability to develop biofilms that harbor virulence-associated genes, which represent a serious threat to human health and food safety.