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Pathogens and other contaminating micro-organisms may survive the normal cleaning and sanitizing procedures followed with parlours and milking equipment. Many do so by being capable to form biofilms. The concern is that biofilm-assisted survival appears to be on the increase. The two studies discussed below give examples of what can be encountered. In the first study, Dr J.L. Gonçalves and co-workers studied biofilm development by a micro-algae isolated from subclinical mastitis cases and then measured the capability of sanitizing agents to control it. Their results were published in the Journal of Dairy Science, Volume 98 of 2015, pages 3613 to 3621, with the title: Biofilm-producing ability and efficiency of sanitizing agents against Prototheca zopfii isolate from bovine subclinical mastitis. In the second study, Dr A. Latorre and colleagues investigated whether the food pathogen listeria monocytogenes can survive in milking equipment on the farm through biofilm development. Their study was also published in the Journal of Dairy Science, Volume 93 of 2010, pages 2792 to 2802, using the title: Biofilm in milking equipment on a dairy farm as a potential source of bulk tank milk contamination with Listeria monocytogenes.
The objectives of the Gonçalves study were firstly, to evaluate the capacity of the micro-algae Prototheca zopfii (isolated from subclinical mastitis cases) to form biofilms, and secondly, to investigate the resistance of these isolates to sanitizing agents. Ten isolates of P. zopfii from cows with subclinical mastitis (somatic cell count, 200 000 cells per mL), distributed over five dairy farms, were evaluated for their capacity to form biofilms at respectively 25°C and 37°C. The micro-algae were isolated from milk samples by microbiological culture and analyzed by gene sequencing. Biofilm development was observed by scanning electron microscopy. The resistance to sanitizing agents was assessed with the biofilm-forming P. zopfii isolates. They were subjected to three sanitizers: peracetic acid, sodium hypochlorite, and iodine solution. To evaluate resistance to the sanitizers, the biofilm isolates were exposed to decreasing concentrations of the sanitizing agents (20, 10, 5, 2.5, 1.25, 0.625, 0.312, 0.156, 0.078, 0.039, and 0.019 g per L). After inoculating the isolates, all concentrations were evaluated at incubation periods of 24, 48 and 72 hours to assess the effect of incubation time on the sanitizing capability.
The results did not show a clear trend due to temperature. However, all P. zopfii isolates had the capacity to form biofilms on stainless steel. The longer the incubation period of the P. zopfii isolates at different dilutions, the greater the concentrations of sanitizer needed to prevent growth of the micro-algae under the tested conditions. Both the effect of sanitizer and time of incubation (24, 48, and 72 hour) were significant. The isolates were sensitive to peracetic acid, sodium hypochlorite and iodine solution after 24 hours of incubation at the higher concentrations. Of the tested sanitizers, peracetic acid had the greatest efficiency against P. zopfii. In conclusion, although the sanitizers apparently were fairly effective, the concern is that this micro-algae, which are associated with subclinical mastitis, are capable of biofilm development, which obviously will contribute to their persistence in a milking and dairy environment.
The objective of the Latorre study was to assess the presence of a Listeria monocytogenes-containing biofilm in milking equipment as a potential source of bulk tank milk contamination on dairy farms. Samples were collected on four occasions and analyzed for the presence of the organism from the milking equipment, the parlour and storage room floors, bulk tank milk, and in-line milk filters. Pieces from milk meters and rubber liners were obtained to visually assess the presence of a biofilm using scanning electron microscopy.
A total of 15%, 25% and 6% samples were positive for L. monocytogenes in the first, second, and third sample collection period, respectively. Six persistant types of 15 L. monocytogenes isolates were obtained from milking equipment, parlour, bulk tank milk and milk filters, one of which was highly predominant. Scanning electron microscopy of samples from the bottom cover of two milk meters showed that individual and clusters of L. monocytogenes bacteria were mainly associated with surface scratches. The presence of a bacterial biofilm of the organism was also observed on the bottom covers of the two milk meters. Since L. monocytogenes is a dangerous food pathogen, prevention of the establishment of such biofilms in milking equipment is crucial in fulfilling the requirement of safe, high-quality milk.