The influence of pig carcass processing of the efficacy of sponge swab sampling

The efficacy of different methods of sampling have been widely compared in the literature. Whilst it is recogn1sed that swabbing and sponging leave a residual bacterial population, the levels that are left are difficult to evaluate and may be Influenced by other factors such as changes to the skin due to processing. In this Food Standards Agency funded study we have used bacterial bioluminescence as a visual marker of the presence of bacteria to evaluate the efficacy of different sampling methods on the removal of bacteria. Pig skin was spiked with a strain of E. coli or Salmonella Typhimurium made bioluminescent by the introduction of the luxCDABE genes from Photorhabdus lummescens on a plasm1d construct. Samples were visualized under a light sensitive camera before and after sponging or swabbmg and the levels of the bacteria removed evaluated. Methods compared were agitated spong•ng, using cellulose acetate sponges, aga1nst traditional sponging and a double-swabbing techmque, us1ng cotton tipped bud swabs. Results md1cate that damage to skm can lead to 'hot spots' of contamination, where residual bacteria are not easily removed by further physical abrasion. Introduction Microbiological sampling and testing of carcasses has been introduced in many countries to verify that HACCP schemes effectively control plant process1ng. Whilst many studies have compared the efficiency of different sampling methods (excision, sponging, wet-dry) (Hutchison et al., 2005; Pepperell et al , 2005), few stud1es have been undertaken on the effic1ency of alternative spongesampling methods. The UK Food Standards Agency (FSA) requires that Salmonella sampling of carcasses can only be undertaken using sponges These are seen as easier to use, particularly on a movmg line, less affected by operator vanabllity and as cost effective because only one set of sampling consumables is requ1red for all of the statutory tests. For testing us1ng sponges the recommended approach is to agitate the sponge by moving 1t by a few centimetres using a side-toside movement (Anon, 2006). Here we evaluate the efficacy of agitated spongmg against a techn1que 1n which multiple sponge passes are made through a delineated area and agamst wetdry swabbmg w1th cotton-tipped swabs. To allow the removal of bactena to be momtored easily, we have spiked pork nnd w1th Escherichia coli or Salmonella Typhimurium engineered to carry the lux genes mak1ng the bactena b1olum1nescent The presence of such bactena on a surface can then tJe viewed using a hght sens1tive camera. Materials and methods Samplmg Sponge sampling was earned out usmg cellulose acetate sponges. swabbmg was carried out usmg cotton tipped bud swabs Agitated sponge sampling was performed on a section of pork rind over a 10 x 10 em area in a single pass. The sponge was agitated from s1de-to-s1de across the whole area in the fash1on recommended by the FSA. Traditional sponge sampling was performed by rubbmg the sponge firmly across the rind surface with 10 strokes 1n each of the horizontal and vertical directions, with no side-to-s1de agitation Wet-dry swab sampling was undertaken by s 1on 6 ntunlcroblal res• tance Safepork 2007 -Verona (Italy 485 486 rubbing a swab moistened in maximum recovery diluent firmly across the rind surface with 10 strokes in each of the horizontal, vertical and both diagonal directions. Swabs were rolled between the thumb and index finger as they were rubbed across the rind surface. Immediately, after rubbing with the moistened swab, the procedure was repeated within the sample template with a dry swab. Efficacy of carcass surface sampling methods Samples of pork rind were inoculated with an Escherichia coli or Salmonella Typhimurium strain which constitutively express luxABCDE genes from Photorhabdus luminescens on a plasmid construct. Bacteria were inoculated to a final concentration of approximately 1 x 105 cfu mr1• Following inoculation the pork rind was incubated at 3rC for 1 hour so that the bacteria could adhere to the pork rind surface. Before and after sampling, photographs were taken of the skin and sponge/swab using a Night Owl CCD camera (EG & G Berthold, Bad Wildbad, Ger.). Two minute integration times were used.