M.R. Adams

Determination of survival, identity and stress resistance of probiotic bifidobacteria in bio‐yoghurts

Aims:  To determine the level of bifidobacteria in bio‐yoghurts in the UK, identify the species, and compare the resistance of common Bifidobacterium spp. to acidity and oxidative stress.

The magnetic immuno-polymerase chain reaction assay for the detection of Campylobacter in milk and poultry

L. DOCHERTY, M.R. ADAMS, P. PATEL AND J. McFADDEN. 1996. A rapid and sensitive technique, based on the magnetic immuno‐polymerase chain reaction assay (MIPA), was developed for the detection of Campylobacter jejuni in milk and chicken products. Target bacteria are captured from the food sample by magnetic particles coated with a specific antibody and the bound bacteria then lysed and subjected to PCR. The MIPA could detect 420 cfu g‐1of chicken after 18 h, 42 cfu g‐1after 24 h, and 4.2 cfu g‐1after 36 h enrichment. For artificially contaminated milk 63 cfu ml‐1could be detected after 18 and 24 h and 6.3 cfu ml‐1after 36 h enrichment.

Enhanced inactivation of Listeria monocytogenes by nisin in the presence of ethanol

Aims: The effect of combinations of nisin and ethanol on the survival of Listeria monocytogenes was investigated.

The role of surface physicochemical properties in determining the distribution of the autochthonous microflora in mineral water bottles

Investigation of the distribution of the viable autochthonous microflora in three brands of 1–2‐month‐old bottled mineral water showed that 1·8 × 104 ( s. e. m. 8·9 × 103, n = 5) to 1·2 × 105 ( s. e. m. 1·3 × 104, n = 5) cfu ml−1 were planktonic cells while 11 ( s. e. m. 4, n = 5)–632 ( s. e. m. 176, n = 5) cfu cm–2 were found in the biofilm. The biofilm represented between 0·03 and 1·79% of the total viable microbial population in the 1·5 litre bottles studied. Scanning electron microscopy studies showed that the cells adhering to the polyethylene terephthalate (PET) bottles were predominantly rod‐shaped, sparsely distributed over the surface. In contrast, the cells adhering to the high density polyethylene (HDPE) caps were found to be mainly clumps of coccoid cells, suggesting that the bottle may provide different microhabitats for different microfloras. Large‐scale roughness, such as that observed as lettering inside the cap (average height (z) = 93 μm) was associated with a 46‐fold increase in cell numbers. Increased small‐scale roughness, as measured by atomic force microscopy on PET and HDPE surfaces (average roughness (Ra) = 5–551 (nm), showed no correlation with adhesion. Investigations of surface hydrophobicity by the sessile drop technique showed that contact angles (u) were greater on the HDPE caps (u = 89–96°) than on the PET surfaces (u = 69–80°). However, no correlation was found between contact angle and attached cell numbers. Measurements of surface electrostatic charge by streaming potential showed that the PET carried an overall negative charge, measuring –15·9 to –16·6 mV in mineral water. No significant change in charge occurred when the monomer composition of the PET was altered. It was concluded that surface roughness, in particular the scale of surface topographical features, is the most important physicochemical surface characteristic determining the distribution of the autochthonous microflora in mineral water bottles

Transient sensitivity to nisin in cold-shocked Gram negatives.

Rapid chilling in the presence of nisin caused a dose‐dependent reduction in the populations of several Gram‐negative bacteria, despite the fact that appreciable structural injury to the outer membrane was not detected. Pseudomonas aeruginosa was most affected, followed by Pseudomonas fragi, Salmonella enteritidis PT4, PT7 and Escherichia coli, respectively. Addition of nisin after the chilling treatment had no effect. The results are ascribed to a transient susceptibility caused by phase changes in the lipids associated with the outer membrane, which are rapidly reversed when the cells return to higher temperatures. Combinations of chilling shock, nisin and EDTA gave much lower reductions of Salmonella and Pseudomonas on chicken skin in comparison with broths. This is attributed to a buffering of the temperature shock experienced by adherent bacteria and binding of the nisin by food particles.

The effect of pH, acidulant and temperature on the survival of Yersinia enterocolitica

The survival of Yersinia enterocolitica at sub‐optimal temperatures (0–23°C) and growth inhibitory pH values, achieved using a range of acidulants, was investigated. At a given pH, survival was greater the lower the temperature. Sulphuric and citric acids had lower bactericidal activity than acetic and lactic acids and in nearly all cases where the four acids could be compared at the same pH the order of bactericidal activity was acetic > lactic > citric > sulphuric. Attempts to model this behaviour by a negative square root relationship gave good correlation coefficients for plots of the square root of death rate against temperature at different combinations of pH and acidulant but so too did several other functions of death rate. The high coefficient of variation for T0 determined from square root plots prevented construction of a combined temperature/pH model similar to that already described for growth.

Comparison of a quadratic response surface model and a square root model for predicting the growth rate of Yersinia enterocolitic

Polynomial equations, relating the growth rate of Yersinia enterocolitica to temperature (0–25°C) and pH (4.5–6‐5) in a liquid medium were constructed for four different acidulants. The logarithm of the time for a 100‐fold increase in bacterial numbers could be represented by a quadratic response surface function of pH and temperature. The interactions between pH and temperature on growth rate were found to be additive. Values for a 2 log cycle increase in growth derived from the model were in good agreement with experimental values. Predictions from the quadratic model and from a square root model were compared with experimental values in laboratory media and UHT milk. The mean square error (MSE) for the quadratic response surface model was smaller than that for the square root model in 81% of cases. In UHT milk the square root model increasingly underestimated growth rate, as the temperature decreased and would ‘fail dangerous’ if used for predictive purposes. This indicated that the response surface model is more reliable for predicting the growth of Y. enterocolitica under conditions of sub‐optimal temperature and pH.

An investigation of the presence of ultramicrocells in natural mineral water

The presence of ‘ultramicrocells’ in natural mineral water, capable of passing through a 0·2 μm filter, has been demonstrated. Filters allowing the greatest proportion of viable (culturable) cells to pass ranked in the order, 0·4 μm polycarbonate (5·02%) > 0·2 μm polycarbonate (0·02%) Ð0·45 μm cellulose nitrate (0·02%) > 0·2 μm cellulose acetate (< 0·002%). Following incubation for 4d at 22 °C, viable counts in filtered mineral water increased from < 2–8·7 × 102 cfu ml−1–2·8 × 104–1·9 × 106 cfu ml−1. Successive filtration/incubation cycles of mineral water increased the proportion of cells passing through a 0·2 μm cellulose acetate filter from < 0·003% to 0·11% and 0·69%, suggesting selection for ‘ultramicrocells’. Cells isolated from this process and grown on liquid R2A medium were thin, Gram‐negative rods, of 0·15–0·40 μm wide and 0·50–6·20 μm long. Membrane filtration techniques used for pathogen detection in mineral waters will not retain all the cells present. If pathogens are able to form ultramicrocells, these may go undetected.

Detection of C. jejuni in Milk and Poultry Using the Magnetic Immuno-Polymerase Chain Reaction Assay

C. jejuni has become the most commonly reported cause of gastro-enteritis worldwide1, 2, 3 and is commonly found as a commensal in poultry, swine, sheep, cattle, dogs, cats, and a number of wild birds and rodents.4,5