A New Rapid Quantitative Method for Detection of Aerobic Plate Count from Environmental Hygiene Swabs

Introduction: There is increased emphasis in food production facilities in maintaining and improving hygiene standards while also reducing downtime.  This has prompted the development for rapid methods that would provide results within 8-12 hours (production cycle) such that corrective action can be taken and product can be released in a timely manner.  Some technologies (ATP bioluminescence) meet the burden of generating contamination results in a shorter time. They do not correlate effectively to bacterial load.  It has been demonstrated that having an environmental swabbing program in place reduces the chance of a foodborne outbreak.

Purpose: This study was conducted to evaluate a new oxygen-sensing method for the quantitative detection of aerobic bacteria from inoculated stainless steel surfaces with environmental hygiene swabs compared to the standard reference method.

Methods: A cocktail containing 0.1 ml of overnight cultures of Escherichia coli, Enterobacter faecium, Salmonella Enteritidis, Listeria monocytogenes, Pseudomonas aeruginosa and Staphylococcus aureus was used to inoculate stainless steel surface with 100 µl of a 10², 10⁴ and 10⁶ dilution.  The surfaces and dilutions were conducted in triplicate.  The inoculated surfaces were allowed to dry for 18h.  They were then swabbed with both a cotton swab and a nylon swab.  The swabs were placed into 15 ml sensor vial and 10 ml of tryptic soy broth (TSB) was added.  The vials were then placed in the system.  Concurrent plate counts were performed for quantitative comparison.

Results: Two separate trials were run with correlation coefficients of plate counts to time-of-results (oxygen depletion) for the cotton swabs were 0.96 and the nylon swab were 0.84.  Results for the oxygen-sensing system showed a typical time-to-result of less than 12 hours at a bacterial load of 10 CFU per swab compared to 48h for the plate count method.

Significance: This new oxygen-sensing method would provide both quantitative and rapid (within a production cycle) results in determining the effectiveness of the sanitation program in a food facility.  In addition, it also allows the release of product in a less than one day versus waiting for micro results which take 24-48 hours.