Microbial Load, Prevalence and Antibiograms of Salmonella and Shigella in Lettuce and Green Peppers

BACKGROUND:   Human foodborne infections traditionally are acquired via ingestion of foods of animal origin.  Fresh fruits and vegetables have now emerged as new and major vehicles for the transmission of zoonotic diseases. Irrigation with poor-quality water and use of raw animal manure for fertilizer can increase the threat of contamination of fruits and vegetables.  The objective of this study was to investigate the microbial quality of fresh salad vegetables, lettuce and green pepper, and to assess the prevalence and antibiotic resistance of Salmonella and Shigella spp. isolated from lettuce and green pepper.      

METHODS:   Samples of lettuce and green peppers were purchased from different outlets in Addis Ababa and analyzed for their load of various microbial groups and flora analysis was conducted following standard microbiological methods.  The presence of Salmonella and Shigella and their antibiotic resistance was also determined.

RESULTS:   Over 90% of the vegetable samples had aerobic mesophilic counts of ≥ log 6 cfu/g. About 97% of the lettuce and 58% of the green pepper samples had enterobacteraceae counts of ≥ log 5 cfu/g. Coliforms were encountered at counts ≥ log 4 cfu/g in 48% and 35% of lettuce and green pepper samples, respectively. Over 80% of vegetable samples harbored staphylococci with counts ranging from log 4 to log 6 cfu/g. More than 88% of lettuce and 18% of green pepper samples had yeast and mold counts ≥ log 4 cfu/g. The aerobic mesophilic flora of the vegetable samples was dominated by Bacillus and Micrococcus spp. Salmonella was isolated from eight (10%) samples. Shigella was also isolated from 24 (30%) samples. All of the Salmonella and 97% of Shigella isolates showed resistance to penicillin. Ampicillin resistance was observed in 42% of Salmonella and 79% of Shigella isolates. Multiple drug resistance (MDR) was seen in 8 and 24 isolates of Salmonella and Shigella isolates.

CONCLUSIONS:   Majority of lettuce and green pepper samples in this study had high microbial load, in some cases, even MDR pathogens were isolated. As salad vegetables don’t get further heat treatment thorough washing, longer exposure of vegetables to food grade chemicals is recommended to kill pathogens and significantly reduce microbial load.