Surveillance of potentially hazardous bacteria in milk and vegetable produce : bacterial content and antibiotic resistance

Abstract

The community and employees are adversely affected through consumption and exposure to hazardous bacteria. Often cause-related illness can be of a transient nature with self-limiting effects that can be serious, or even lifethreatening or fatal. The highest possible quality of milk should therefore reach the consumer. Not only microbiological counts, but the reliable detection of pathogens and antibiotic residue should also be part of routine analysis. The microbiological status of milk samples from all categories collected in the Bloemfontein area during 1997 and 1998 were generally considered unsatisfactory. Some of the milk samples from individual cows tested in this study were of a good hygienic quality. However, as soon as milk lots were combined as a batch, the quality declined remarkably. The majority of the milk sold at depots did not conform to one or more of the standards as specified by law. A total of 55 milk samples were collected. Of a total of 20 raw batch samples tested, 85% did not conform to the standards for raw milk, whilst 82.6% of 23 depot and industrially pasteurised milk samples did not conform. Of concern was the fact that 5/19 depot milk samples failed all three microbiological count standards. In this study, apart from Staphylococcus aureus, Listeria species and two ~haemolytic streptococci, no other pathogens were detected. The Listeria isolation method is well-developed and consists of two enrichment stages. Many potentially promising pathogens were eventually identified as opportunistic pathogens, such as Enterobacter sakazakii and Ent. agg/omerans. Polymerase chain reaction (PCR) is a very convenient method to be used for the identification of organisms and was used as alternative to conventional microbiological methods for pathogen detection. Milk, however, is not an easy medium to incorporate into the PCR system. Difficulty was experienced in obtaining good reproducible target DNA, and the PCR product sizes were not always as expected. It will be necessary to test local strains and to continuously correlate and update PCR performance. In this study it was found that certain factors in milk had an influence on the PCR results. Freezing and thawing of a milk sample adversely affected PCR results. The Dynabead® DNA concentration concept, which is vital for the detection of low cell numbers, performed very well, but was also affected by the freezing and thawing process. The conventional microbiological and PCR results concerning L. monocytogenes were in accordance. The use of PCR for the detection of Campy/obacter and Brucella appeared promising. Although PCR technology can be fraught with inherent problems, it remains the most comprehensive and convenient option for future rapid detection/identification of food borne pathogens. Problems experienced with antibiotic administration for a variety of reasons have lead to the misuse of antibiotics by farmers Lack of knowledge and the use of growth promoters could be reasons for the incidence of antibiotic-resistant strains in the environment. Gram-positive and gram-negative strains were selected from milk, vegetable and farm environmental samples and investigated for antibiotic-resistant genes. The resistance profiles of staphylococci isolated from raw and pasteurised milk during 1997 and 1998 showed S. aureus resistance to three or more antibiotics with streptomycin, tetracycline and cephalosporin resistance evident. The majority of antibiotic-resistant coagulasenegative staphylococci (CNS) showed resistance to ciprofloxacin, streptomycin and oxacillin. Milk is one of the most nutritious products available at an affordable price. In a country where the majority of the community lives in poverty, it is a product of critical importance. Milk that is bought directly from the farmer or at depots is usually cheaper than those sold at shops. In this study it was clear that milks did not comply to microbiological regulations. Pathogens and antibiotic resistant strains were detected. Consumers have the right to receive milk of good quality, which is free from pathogens and antibiotic residue.