Validation of the Hygicult® E Dipslides Method in Surface Hygiene Control: A Nordic Collaborative Study

A collaborative study with Enterobacteriaceae was conducted to validate Hygicult® E dipslides by comparison with violet red bile glucose agar (VRBGA) contact plates and swabbing, using stainless steel surfaces artificially contaminated with microbes at various levels. Twelve laboratories participated in the validation procedure. The total number of collaborative samples was 108. The microbial level in each sample was assessed in triplicate by using the 3 above-mentioned methods. No Enterobacteriaceae were used at the low inoculation level. At the middle inoculation level, the percentages detached from the test surfaces were 16.6 with the Hygicult E method, 15.3 with the contact plate method, and 14.6 with swabbing; at the high innoculation level, the percentages were 14.5, 15.8, and 9.8, respectively. The percentage of acceptable results after the removal of outliers was 97.2. Repeatability relative standard deviations ranged from 33.4 to 44.9%; reproducibility relative standard deviations ranged from 45.2 to 77.1%. The Hygicult E dipslide, VRBGA contact plate, and swabbing methods gave similar results at all 3 microbial levels tested: <1.0 colony-forming units (CFU)/cm2 at the low level, 1.2–1.3 CFU/cm2 at the middle level (theoretical yield 8.0 CFU/cm2), and 1.2–2.0 CFU/cm2 at the high level (theoretical yield 12.5 CFU/cm2).

Validation of the Microbiological Methods Hygicult Dipslide, Contact Plate, and Swabbing in Surface Hygiene Control: A Nordic Collaborative Study

A collaborative study on total aerobic bacterial count was conducted to validate the Hygicult® TPC dipslide against contact plates and swabbing, using stainless-steel surfaces artificially contaminated with different microbes at various levels. Twelve laboratories took part in the validation procedure. The total number of collaborative samples was 108. The microbial level in each sample was assessed in triplicate using the 3 above-mentioned methods under 3 different incubation conditions (at 25 ± 1°C for 48 and 72 h and at 30 ± 1°C for 48 h). Surface sampling methods detached 25–30% at the lowest (theoretical yield, 1.4 cfu/cm2), 18–20% at the middle (theoretical yield, 10.7 cfu/cm2), and 16–21% at the highest (theoretical yield, 43.6 cfu/cm2) levels of microbes from the test surfaces. The percentage of acceptable results after removing outliers was 89%. Repeatability standard deviations ranged from 27.2 to 74.6% and reproducibility standard deviations ranged from 42.1 to 97.5%. There were no significant differences between results obtained at different incubation temperatures (25 and 30°C) or incubation times (48 and 72 h) for all 3 methods. The Hygicult TPC dipslide, contact plate, and swabbing methods gave similar results at all 3 microbial levels tested: 0.35–0.43 cfu/cm2 at the lowest level, 1.9–2.2 cfu/cm2 at the middle level, and 7.1–9.1 cfu/cm2 at the highest level.

Acid and heat tolerance of persistent and nonpersistent Listeria monocytogenes food plant strains

AIMS

Acid and heat tolerance of 17 persistent and 23 nonpersistent Listeria monocytogenes strains, recovered from three meat-processing plants, were investigated.

METHODS AND RESULTS

The isolates were genotyped by pulsed-field gel electrophoresis and categorized into persistent strains according to the frequency of the strain and duration of the contamination. The persistent and nonpersistent strains were challenged to acidic conditions (pH 2.4 for 2 h, 1 mol l(-1) HCl were used to acidify the suspension) and to heat (55 degrees C for 40 min) to receive a reduction in cell count. Listeria monocytogenes strains showed large variation in acid tolerance (over 6 log units) and in heat tolerance (3 log units). The persistent strains showed higher tolerance to acidic conditions than the nonpersistent strains (Student's t-test, P = 0.02), but significant differences in heat tolerance between persistent and nonpersistent strains were not observed.

CONCLUSIONS

The results indicate that acid tolerance may have an effect on the persistence of L. monocytogenes contamination.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study highlights the fact that there are great differences in acid and heat tolerances between L. monocytogenes strains, and the preventive measures should be designed to be effective against the most tolerant strains.

Listeria monocytogenes contamination pattern in pig slaughterhouses

Ten low-capacity slaughterhouses were examined for Listeria by collecting a total of 373 samples, of which 50, 250, and 73 were taken from carcasses, pluck sets, and the slaughterhouse environment, respectively. Six slaughterhouses and 9% of all samples were positive for Listeria monocytogenes. Of the samples taken from pluck sets, 9% were positive for L. monocytogenes, the highest prevalence occurring in tongue and tonsil samples, at 14% and 12%, respectively. Six of 50 (12%) carcasses were contaminated with L. monocytogenes. In the slaughterhouse environment, L. monocytogenes was detected in two, one, one, and one sample originating from the saws, drain, door, and table, respectively. Carcasses were contaminated with L. monocytogenes in those two slaughterhouses, where the mechanical saws, used for both brisket and back splitting, were also positive for L. monocytogenes. A total of 58 L. monocytogenes isolates were characterized by pulsed-field gel electrophoresis typing. The isolates were divided into 18 pulsotypes, 15 of which were detected in pluck sets. In two slaughterhouses, where the carcasses were contaminated with L. monocytogenes, the same pulsotypes were also recovered from splitting saws. In addition, identical pulsotypes were recovered from pluck sets. Our findings indicate that L. monocytogenes of tongue and tonsil origin may contaminate the slaughtering equipment that may in turn spread the pathogen to carcasses. Thus, it is of the utmost importance to follow good manufacturing practices and to have efficient cleaning and disinfection procedures to prevent equipment being contaminated with L. monocytogenes.