Evaluation of Biofilm Production and Antibiotic Resistance/Susceptibility Profiles of Pseudomonas spp. Isolated from Milk and Dairy Products

Dairy-borne <i>Pseudomonas</i> spp., known for causing spoilage, may also exhibit antibiotic resistance and form biofilms, enhancing their persistence in dairy environments and contaminating final products. This study examined biofilm formation and antibiotic resistance in 106 <i>Pseudomonas</i> spp. strains isolated from milk, whey, and spoiled dairy products. Phylogenetic analysis (based on partial <i>ileS</i> sequences) grouped most strains within the <i>P. fluorescens</i> group, clustering into the <i>P. fluorescens</i>, <i>P. gessardii</i>, <i>P. koorensis</i>, and <i>P. fragi</i> subgroups. Biofilm formation in polystyrene microplates was assessed at 6 °C and 25 °C by crystal violet staining. After 48 h, 72% and 65% of <i>Pseudomonas</i> strains formed biofilms at 6 °C and 25 °C, respectively, with higher biomass production at 6 °C. High biofilm producers included most <i>P. fluorescens</i>, <i>P. shahriarae</i>, <i>P. salmasensis</i>, <i>P. atacamensis</i>, <i>P. gessardii</i>, <i>P. koreensis,</i> and <i>P. lundensis</i> strains. The <i>adnA</i> gene, associated with biofilm formation, was detected in 60% of the biofilm producers, but was absent in <i>P. fragi</i>, <i>P. lundensis</i>, <i>P. weihenstephanensis</i>, and <i>P. putida</i>. Antibiotic susceptibility was tested using the disk diffusion method. All strains were susceptible to amikacin and tobramycin; however, 73% of the strains were resistant to aztreonam, 28% to imipenem and doripenem, 19% to ceftazidime, 13% to meropenem, and 7% to cefepime. A multiple antibiotic resistance index (MARI) > 0.2 was found in 30% of the strains, including multidrug-resistant (<i>n</i> = 15) and extensively drug-resistant (<i>n</i> = 3) strains. These findings highlight <i>Pseudomonas</i> spp. as persistent contaminants and antibiotic resistance reservoirs in dairy environments and products, posing public health risks and economic implications for the dairy industry.