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Prevalence of Pathogenic Bacteria in Common Salad Vegetables of Dhaka Metropolis

FARJANA RAHMAN
Department of Microbiology, Stamford University Bangladesh, 51 Siddeswari Road, Dhaka

RASHED NOOR
Department of Microbiology, Stamford University Bangladesh, 51 Siddeswari Road, Dhaka

Abstract/ Executive Summary:

Microbial quality of common salad vegetables (viz. carrot, cucumber, tomato and lettuce) collected from Dhaka metropolis was analysed to detect the presence of bacterial pathogens. The occurrence of huge numbers of fecal coliforms (1.0×10⁴ - 4.09×10⁶ cfu/g), Escherichia coli (1.0×10⁴ - 5.0×10⁸ cfu/g), Staphylococcus aureus (2.0×105 - 5.95×10⁷ cfu/g), and Listeria spp. (1.5×10⁶ - 6.5×10⁷ cfu/g) were detected in all the tested samples. Interestingly, occurrence of viable but non-culturable (VBNC) bacteria was also noticed.

Keywords: Raw vegetables, Pathogens, Fecal coliforms, Health risks

Location: Dhaka

Start Date:

End Date:

Program Area: Pest Management

Commodity:

Objectives:

An attempt was taken to assess the bacteriological quality, particularly pathogenic bacteria of fresh salad vegetables collected from several retail shops in Dhaka city.

Methodology:

A total of four different samples of common salad vegetables collected from different local markets and super shops of Dhaka City were used for microbiological analysis. All the samples were collected according to the method suggested by American Public Health Association (APHA 1998). A portion of 25 g of each vegetable sample was aseptically weighed and buffer peptone water was added to make the final volume 100 ml. Serial dilutions were made up to 10-5 for plating (Cappuccino and Sherman 1996). For isolation of fecal coliforms such as Escherichia coli, Klebsiella spp., Staphylococcus aureus, and Listeria spp., MFC agar, MacConkey agar, Manitol Salt Agar (MSA) and Listeria isolation media were used. After incubation at 37°C except for fecal coliforms (44.5°C) for 24 h, observations were made for the characteristic colonies (Cappuccino and Sherman 1996). The presence of E. coli was further confirmed by the appearance of bluish-black colonies with the production of green metallic sheen on Eosin-Methylene Blue (EMB) agar medium. During isolation, enrichment was performed for Salmonella, Shigella (in selenite cystine broth) and for Vibrio spp. (in alkaline peptone water) in order to avoid the false negative results due to the possibility of these bacteria to be in stressed condition or in the viable but non-culturable (VBNC) state in the samples, thereby decreasing the chance of their cultivability in the isolation media (Oliver 2005, Colwell 2000). For Salmonella and Shigella spp., Xylose Lysine Deoxycholate (XLD) plates were used while for Vibrio spp., Thiosulfate Citrate Bile Salt Sucrose (TCBS) agar plates were used. After incubation at 37°C for 24 h, characteristic colonies were detected and enumerated. Attempt was also taken to isolate Clostridium spp. from the vegetable samples (Francis et al. 1999). Major biochemical tests such as Triple Sugar Iron (TSI), Motility Indole Urease (MIU), Methyl-Red (MR), Voges-Proskauer (VP), Citrate Utilization, Catalase test and Oxidase tests were carried out following the standard methods (Alfrad 2007, Cappuccino and Sherman 1996). All the experiments were performed in triplicate and the results were reproducible.

Source of Information: Bangladesh J. Bot. 41(2): 159-162, 2012 (December)

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

An important aspect of this study was the isolation of Listeria spp. from the salad vegetables which has been reported to represent an imperative risk to public health globally. Thus, the present study adds new insight to the existing knowledge on the microbiology of the commonly consumed fresh salads in Bangladesh. Another interesting facet from this study lies on the existence of VBNC state of the pathogens in the salad vegetables. Extensive identification of such cells would further unveil the factual implications of such bacteria on food borne epidemic outbreaks.

Knowledge Management: Journal