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Title: RESUSCITATION OF ACID-INJURED SALMONELLA IN ENRICHMENT BROTH, IN APPLE JUICE AND ON THE SURFACES OF FRESH-CUT CUCUMBER AND APPLE

Author
item LIAO, CHING HSING
item FETT, WILLIAM

Submitted to: Letters in Applied Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/3/2005
Publication Date: 11/1/2005
Citation: Liao, C., Fett, W.F. 2005. Resuscitation of acid-injured salmonella in enrichment broth, in apple juice and on the surfaces of fresh-cut cucumber and apple. Letters in Applied Microbiology. 41:487:492.

Interpretive Summary: Disease-causing (pathogenic) bacteria associated with foods can be killed, injured, or unaffected when exposed to physical or chemical treatments. Since injured bacteria may not be detectable by conventional methods, we have identified the liquid broth most suitable for recovery and growth of Salmonella pathogen injured by acetic acid (an active ingredient of vinegar) treatment. We also examined the recovery of bacterial injury in apple juice and on cut surfaces of apple and cucumber fruits at different temperatures. Results indicated that recovery of acid-injured bacteria occurred at elevated temperatures, but not at refrigeration temperature, on cut surfaces of cucumber fruits, but not on cut surfaces of apple fruits, and in non-selective broth, but not in selective broth containing antimicrobial agents. To improve the detection of injured pathogens on fresh produce, samples should be incubated in non-selective broth before placing them in selective broth. We demonstrated that the Universal Pre-Enrichment Broth was the most effective among the three non-selective broths tested for resuscitation of acid-injured Salmonella. This finding will assist regulatory agencies and food industry to detect low levels of injured bacteria on minimally processed foods.

Technical Abstract: For detection of a low number of pathogens in processed foods, an enrichment step is frequently used to resuscitate any injured bacteria and to increase the concentration of targeted pathogens to a level detectable by cultural or PCR methods. The objectives of this study were to determine the adequacy of selective and non-selective broth media for resuscitation of acetic acid (AA)-injured Salmonella and to investigate the recovery of AA-injured Salmonella in apple juice, phosphate buffered saline (PBS), and on fresh-cut surfaces of fruits. Recovery of AA-injured Salmonella occurred in three non-selective enrichment media including lactose broth (LB), universal pre-enrichment broth (UPB) and buffered peptone water (BPW), but not in selective tetrathionate (TT) broth, phosphate buffered saline (PBS), and apple juice. AA-injured Salmonella appeared to recover significantly (p<0.05) faster in UPB than in LB or BPW. Recovery also occurred on cut surfaces of cucumber fruits, but not on cut surfaces of apple fruits, and at 20 or 35 deg C, but not at 4 deg C. These results indicate that acid-injured Salmonella can resuscitate and proliferate on fruit surfaces and are a safety hazard of fresh fruits stored at elevated temperatures. Direct enrichment of the sample in TT, which may inhibit the recovery of injured cells, should be avoided. To improve the detection of acid-injured Salmonella on fresh produce, enrichment of the sample in UPB instead of LB or BPW is recommended.