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Bacteriological contamination of farm and market kale in Nairobi and its ENvirons

Abstract

This study aimed at determining the microbiological safety of kale (Brassica oleracea Acephala), a green leafy vegetable produced and sold in Nairobi. The assessment was carried out on kale and water (used for irrigation on farms and washing/refreshing of kale at the markets); the water being assessed as one of the sources of contamination for the kale. This was a cross-sectional study in which kale samples were collected from randomly selected farmers and traders. Data on farming and trading practices was collected using questionnaires administered through personal interview.

 Samples were collected from selected peri-urban farms in Athi River, Ngong, and Wangige. They were also collected from traders from wet markets in Kawangware, Kangemi and Githurai, a supermarket and a high-end specialty store all within Nairobi city. Coliform counts, plus Escherichia coli and Salmonella species isolation and characterization were done. Isolated Salmonella were further analyzed for virulence, pathogenicity and antibiotic sensitivity. This study was necessary since there have been public health concerns over the poor practices in production and distribution of leafy vegetables in Nairobi and its environs.

Mean coliform counts on kale leaves from farms ranged from1.6x105±9.1x104 to  4.0x105±1.3x105 colony forming units per gram (cfu/g) while those from the wet markets ranged from 1.1x106±6.7x105 to 1.1x107±3.0 x106cfu/g. Kale samples from supermarkets had a mean coliform count of 2.7x106 ±5.5x105 cfu/g while those from high-end specialty store were 4.7x105 ±2.3x105 cfu/g. Coliform numbers obtained on kale samples from the wet markets and the supermarket were significantly higher (p<0.05) compared to those from farms. Kale samples purchased from high- end market had similar levels of coliform loads as those from the farms (p> 0.05). Escherichia coli prevalences in samples from farms ranged from 37.7% (6/16) to 81.1% (18/22). Those from the wet markets ranged from 33.3% (6/18) to 62.5% (10/16) while those from supermarkets and high-end specialty store, were 20% (5/25 and 3/15, respectively). Salmonella organisms were detected on 4.5% (1/22) and 6.3% (1/16) of samples collected from farms in Wangige and a market in Kawangware, respectively. It was also detected in 12.5% (1/8) of water samples used for washing/refreshing kale from a market in Kangemi. Fecal coliforms in water used on farms (for irrigation) and in the markets (for washing the vegetables) exceeded levels recommended by World Health Organization (WHO) counts of 103 per 100 milliliters.

Salmonella strains isolated from kale and water were virulent and pathogenic to mice though they exhibited significantly lower (p<0.05) pathogenic characteristics when compared to those from human clinical sources. They also showed resistance to only Penicillin, out of the 9 antibiotics that they were screened for; unlike those from clinical sources which showed multi - drug resistance.

This study has, therefore, demonstrated low bacteriological quality of kale and associated water in Nairobi and its environs. These may be carriers of pathogenic organisms which could cause disease to human consumers. The isolation of Salmonella organisms has further emphasized this, more so since the Salmonella isolates demonstrated resistance to some antibiotics. It is therefore recommended that good farming and handling practices be undertaken to increase the safety of leafy vegetables. Consumers are also advised to cook their vegetables well before consumption.

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