F.M. Nattress

Evaluation of the ability of lysozyme and nisin to control meat spoilage bacteria

The antimicrobials lysozyme, nisin, and mixtures of the two were studied to ascertain their abilities to control the growth of the meat-borne spoilage bacteria, Brochothrix thermosphacta B2 and Carnobacterium sp. 845. The goal was to optimize an antimicrobial for potential use in preservation of fresh meats. Their efficacies were evaluated in APT broth, in a meat juice extract and on cores of lean and fat pork tissue. Both lysozyme and nisin alone as well as mixtures of the two effectively inhibited B. thermosphacta B2 at 250 microg/ml in APT broth, the lowest concentration evaluated, for 10 days at 2 degrees C. In the presence of 500 microg/ml lysozyme, B. thermosphacta B2 grew after 12 days incubation. Only 125 microg of antimicrobial/ml was required to inhibit B. thermosphacta B2 for 27 days at 2 degrees C in pork juice. An estimated surface concentration of 130 microg/cm2 of each of the antimicrobials effectively inhibited B. thermosphacta B2 on inoculated cores of fat and lean pork tissue when the cores were incubated in vacuum packages for 6 weeks at 2 degrees C. In APT broth and in pork juice, lysozyme showed no antimicrobial activity against Carnobacterium sp. 845 at concentrations of 500 and 1000 microg/ml, respectively. Nisin and mixtures of the two antimicrobials inhibited Carnobacterium sp. 845 so that its numbers were at least 3 log units lower than untreated samples after 26 and 27 days incubation for APT and pork juice, respectively. The antimicrobial effect was concentration dependent. On lean pork tissue, numbers of Carnobacterium sp. 845 were significantly lower than untreated samples or samples treated with 195 microg/cm2 lysozyme when 260 microg/cm2 of a 1:3 (w/w) ratio of nisin to lysozyme was introduced to the cores. The inhibitory effect lasted for 14 of 42 days incubation in vacuum at 2 degrees C. On fat tissue, both lysozyme alone and the 1:3 nisin/lysozyme mixture inhibited Carnobacterium sp. 845 for 21 days storage in vacuum at 2 degrees C. On fat and lean tissue, mixtures of nisin and lysozyme would be more effective antimicrobials than either nisin or lysozyme alone.

Origin of contamination and genetic diversity of Escherichia coli in beef cattle.

ABSTRACT The possible origin of beef contamination and genetic diversity of Escherichia coli populations in beef cattle, on carcasses and ground beef, was examined by using random amplification of polymorphic DNA (RAPD) and PCR-restriction fragment length polymorphism (PCR-RFLP) analysis of the fliC gene. E. coli was recovered from the feces of 10 beef cattle during pasture grazing and feedlot finishing and from hides, carcasses, and ground beef after slaughter. The 1,403 E. coli isolates (855 fecal, 320 hide, 153 carcass, and 75 ground beef) were grouped into 121 genetic subtypes by using the RAPD method. Some of the genetic subtypes in cattle feces were also recovered from hides, prechilled carcasses, chilled carcasses, and ground beef. E. coli genetic subtypes were shared among cattle at all sample times, but a number of transient types were unique to individual animals. The genetic diversity of the E. coli population changed over time within individual animals grazing on pasture and in the feedlot. Isolates from one animal (59 fecal, 30 hide, 19 carcass, and 12 ground beef) were characterized by the PCR-RFLP analysis of the fliC gene and were grouped into eight genotypes. There was good agreement between the results obtained with the RAPD and PCR-RFLP techniques. In conclusion, the E. coli contaminating meat can originate from cattle feces, and the E. coli population in beef cattle was highly diverse. Also, genetic subtypes can be shared among animals or can be unique to an animal, and they are constantly changing.

Effects of treatment with lysozyme and nisin on the microflora and sensory properties of commercial pork

A mixture of lysozyme and nisin at a ratio of 3:1 (w/w) and at a surface concentration of approximately 260 microg/cm2 was effective in controlling the growth of lactic acid bacteria, lactic acid bacteria able to grow in the presence of acetate and Brochothrix thermosphacta on naturally contaminated pork loins that were stored in vacuum packages at 2 degrees C for up to 6 weeks. When loins were removed, cut into chops, and displayed in a retail display case, the efficacy of the antimicrobial mixture decreased with increasing display time. At most sampling times, bacterial numbers were lower in treated samples than in untreated samples. The numbers of Enterobacteriaceae were higher in treated samples than in untreated samples. The growth of Enterobacteriaceae may have been reduced as a result of antimicrobial activity of the lactic acid bacteria because when the growth of lactic acid bacteria was inhibited by the antimicrobial treatment, the Enterobacteriaceae were able to grow to higher numbers. Sensory evaluation of the loins showed no difference between treated and untreated samples, but aerobically displayed chops treated with antimicrobial had more prevalent off-odours and reduced odour acceptability than untreated samples.

Genotypic analysis of Escherichia coli recovered from product and equipment at a beef-packing plant

Aims:  To identify sources of Escherichia coli on beef by characterizing strains of the organism on animals, equipment and product at beef‐packing plant.

Molecular typing techniques to characterize the development of a lactic acid bacteria community on vacuum-packaged beef

The development of a community of lactic acid bacteria from vacuum-packaged beef was investigated during a 6-week storage trial at 2 degrees C. The lactic acid bacteria population was monitored by using molecular techniques to identify a random sample of isolates at biweekly intervals during the storage trial. The polymerase chain reaction and a randomly amplified polymorphic DNA technique were used to identify and distinguish populations of lactic acid bacteria that developed during the storage trial. At week 0, the population of lactic acid bacteria was 3.5 log cfu/120 cm2 and by week 6, the population reached a maximum of 7.6 log cfu/120 cm2. A sampling from the week 0 population indicated a mixed community of Lactobacillus curvatus, Lactobacillus sakei and Leuconostoc spp. However, the sampling from week 6 indicated the population composition had changed to one where a single Leuconostoc strain predominated. This strain demonstrated antagonism towards the growth of other lactic acid bacteria isolated during the study. Additionally, the strain inhibited the growth of foodborne pathogens Escherichia coli O157:H7 and Listeria monocytogenes. DNA sequence data from the 16S rRNA gene suggested that the isolate may be a Leuconostoc gelidum strain.

Bacterial contamination of recirculating brine used in the commercial production of moisture-enhanced pork.

In a commercial process for the production of moisture-enhanced pork, boneless pork loins were conveyed through a recirculating injection apparatus, and brine (sodium phosphate, sodium chloride, and lemon juice solids) was pumped into the meat through banks of needles inserted automatically into the upper surfaces of cuts. Brine samples were collected at intervals during the production process and analyzed to determine the total plate count and the numbers of lactic acid bacteria, pseudomonads, Brochothrix thermosphacta, and Enterobacteriaceae. Listeria monocytogenes numbers in the brine were determined using a PCR with primers for the hemolysin gene in combination with a most probable numbers determination. Maximum numbers of bacteria (log CFU/ml) recovered from the brine after 2.5 h of recirculation were as follows: total plate count, 4.50; lactic acid bacteria, 2.99; pseudomonads, 3.95; B. thermosphacta, 2.79; and enterics, 3.01. There was an increase in the number of L. monocytogenes in the recirculating brine with time, reaching a maximum of 2.34 log CFU/100 ml after 2.5 h of moisture-enhanced pork production. Thus, recirculating brines can harbor large populations of spoilage bacteria and L. monocytogenes and are an important source of contamination for moisture-enhanced pork.

Survival of Campylobacter jejuni on beef and pork under vacuum packaged and retail storage conditions: Examination of the role of natural meat microflora on C. jejuni survival

The ability of Campylobacter jejuni ATCC 11168 to survive on beef and pork stored under chilled, vacuum packaged and retail display conditions were examined. In addition, the effect of natural microflora on commercial beef and pork on the survival of C. jejuni under these storage conditions was examined. When sterile cores of beef and pork were inoculated with ∼ 10(5) to 10(6) cfu cm(-2)C. jejuni, and were stored under aerobic or vacuum packaged conditions at -1.5 or 4 °C, its numbers dropped significantly and C. jejuni could not be enumerated by direct plating after 21 d of the 6 wks study. In contrast, survival of C. jejuni on commercial vacuum packaged beef and pork was significantly enhanced, resulting in only 1 log cfu cm(-2) reduction at the end of 6 wks. During 7 d of display in a retail case, numbers of C. jejuni dropped quickly, but could be enumerated by direct plating even after the 7 d. The presence of high numbers of inoculated C. jejuni on beef and pork had no significant effect on the natural microflora numbers compared to uninoculated controls when the meat was stored either in vacuum or in a retail display case. These results show that natural microflora on vacuum packaged meat afford enhanced survival of C. jejuni present on the surfaces of both beef and pork when stored at refrigeration temperatures. Hence, strict hygienic practices or the implementation of decontamination technologies are recommended to ensure safety of meat with respect to this pathogen.

Genetic diversity of Escherichia coli recovered from the oral cavity of beef cattle and their relatedness to faecal E. coli

Aims:  To determine the genetic diversity of generic Escherichia coli recovered from the oral cavities of beef cattle and their relatedness to E. coli isolated from the faeces of cattle during pasture grazing and feedlot finishing.