Reduction, Prevention, and Control of Salmonella enterica Viable but Non-culturable Cells in Flour Food

The processing and storage conditions of flour food inevitably pose environmental stress, which promote bacteria to enter a viable but non-culturable (VBNC) state. The existence of VBNC cells causes false-negative detection in traditional culture-based detection methods, resulting in food quality and safety issues. This study aimed at investigating the influence factors including nutrition, acid, salt, and temperature for the entry into a VBNC state of Salmonella enterica and an efficient detection method. During induction with multi-stress conditions, nutrition starvation antagonizes with low-level acidity. Besides, high-level acidity was considered as an inhibitor for VBNC induction. Four inducers including nutrition starvation, salt stress, low-level acidity, and low temperature were concluded for a VBNC state. In addition, the keynote conditions for S. enterica entering a VBNC state included (i) nutrient-rich acidic environment, (ii) oligotrophic low-acidity environment, and (iii) oligotrophic refrigerated environment. Based on the keynote conditions, the environmental conditions of high acidity (1.0% v/v acetate) with low temperature (-20°C) could successfully eliminate the formation of S. enterica VBNC cells in flour food. In addition, combining with propidium monoazide pretreatment, PCR technology was applied to detect S. enterica VBNC cells. The sensitivity of the PMA-PCR technology was 10⁵ CFU/ml in an artificially simulated food system. The results derived from this study might aid in the detection and control of VBNC state S. enterica in flour food products.

Postharvest intervention technologies for safety enhancement of meat and meat based products; a critical review

Globally, the demand for safe, healthy and nutritious meat and allied products possesses improved taste with extended shelf life is mounting. Microbial safety is among the imperative challenges that prevails in meat products because they provide an ideal medium for the growth of microorganisms particularly pathogenic bacteria. The incidence of these microbes can result quality deterioration of products leading towards food borne diseases when consumed by peoples. Several preservation technologies like chemical and biological interventions are effective to retard or inactivate the growth of micro-organisms most commonly related to food-borne diseases. Despite these, innovative approaches like hydrostatic pressure processing, active packaging, pulse electric field, hurdle approach and use of natural antimicrobials can be deployed to enhance the safety of meat and meat products. The objective of review is to describe the current approaches and developing technologies for enhancing safety of meat and allied meat products.

Acetic acid induces pH-independent cellular energy depletion in Salmonella enterica

Weak organic acids are widely used as preservatives and disinfectants in the food industry. Despite their widespread use, the antimicrobial mode of action of organic acids is still not fully understood. This study investigated the effect of acetic acid on the cell membranes and cellular energy generation of four Salmonella strains. Using a nucleic acid/protein assay, it was established that acetic acid did not cause leakage of intracellular components from the strains. A scanning electron microscopy study further confirmed that membrane disruption was not the antimicrobial mode of action of acetic acid. Some elongated Salmonella cells observed in the micrographs indicated a possibility that acetic acid may inhibit DNA synthesis in the bacterial cells. Using an ATP assay, it was found that at a neutral pH, acetic acid caused cellular energy depletion with an ADP/ATP ratio in the range between 0.48 and 2.63 (p<0.05) that was apparent for the four Salmonella strains. We suggest that this effect was probably due solely to the action of undissociated acid molecules. The antimicrobial effect of acetic acid was better under acidic conditions (ADP/ATP ratio of 5.56 ± 1.27; p<0.05), where the role of both pH and undissociated acid molecules can act together. We concluded that the inhibitory effect of acetic acid is not solely attributable to acidic pH but also to undissociated acid molecules. This finding has implication for the use of acetic acid as an antimicrobial against Salmonella on food products, such as chicken meat, which can buffer its pH.

Resuscitation of acid-injured Salmonella in enrichment broth, in apple juice and on the surfaces of fresh-cut cucumber and apple

AIMS

To investigate the resuscitation of acid-injured Salmonella enterica in selected enrichment broths, in apple juice and on cut surfaces of apple and cucumber slices.

METHODS AND RESULTS

Following exposure to 2.4% acetic acid for 7 min, S. enterica (serovars Mbandaka, Chester and Newport) cells were used to inoculate enrichment broths, phosphate-buffered saline (PBS), apple juice and fruit slices. Injured Salmonella cells resuscitated and regained the ability to form colonies on selective agar (Xylose-Lysine-Tergitol 4) if they were incubated in lactose broth (LB), universal pre-enrichment broth (UPB) or buffered peptone water (BPW), but not in tetrathionate broth, PBS or apple juice. The resuscitation occurred at a significantly (P > 0.05) faster rate in UPB than in LB or BPW. The resuscitation also occurred on the surfaces of fresh-cut cucumber at 20 degrees C, but not at 4 degrees C.

CONCLUSIONS

Acid-injured Salmonella cells resuscitated in nonselective enrichment broths at different rates, but not in selective enrichment broth, apple juice, PBS or on fresh-cut apple.

SIGNIFICANCE AND IMPACT OF THE STUDY

Pre-enrichment of food samples in UPB prior to selective enrichment is recommended. Injured Salmonella cells have the ability to resuscitate on fresh-cut surfaces of cucumber when stored at abusive temperatures.