Assessment of Bacterial Accumulation and Environmental Factors in Sentinel Oysters and Estuarine Water Quality from the Phang Nga Estuary Area in Thailand

Abstract

This study characterized microbiological and chemical contamination of oyster meat and estuarine water in Phang Nga, Thailand. Pooled oyster meats (n = 144), estuarine waters (n = 96) and environmental parameters were collected from March, 2016 to February, 2017, and assessed for levels of total coliforms (TC), fecal coliforms (FC), Escherichia coli (EC), and Vibrio parahaemolyticus (VP), presence of Salmonella and Shigella and levels of heavy metals (Mn, Pb and Cd). The prevalence of TC, FC and EC were in 99.3%, 94.4% and 93.1% of oyster meat and 94.8%, 79.2%, and 78.1% of water, respectively. The average VP levels was 8.5 × 10⁷ most probable number (MPN)/g oyster. Prevalence of Shigella and Salmonella in the pooled oysters were 7.6% and 30.6%, respectively. The dominant Salmonella serovars were Paratyphi B followed by Seremban, and Kentucky. In contrast, the prevalence of Shigella were 27.1%, but Salmonella was not detected in estuarine water. Factors statistically associated with EC accumulation in oyster were level of FC, 7-day average precipitation, temperature, relative humidity, and presence of Salmonella in the sample. The optimal cutoff value of EC to predict Salmonella in oyster was 420 MPN/g. Results indicate this area has relatively safe levels of heavy metals, whereas bacterial contamination was very high for oysters.

Keywords: Escherichia coli; Salmonella; Shigella; Vibrio parahaemolyticus; estuarine water; fecal coliforms; oyster.

Figure 1

Four sampling sites are indicated as black circle dots, where pooled oyster and estuarine water samples were collected from Marui cannal in Phang Nga bay from southern Thailand. Source: Map of Thailand (www.diva-gis.org/gdata).

Figure 2

Number of positive samples for Salmonella spp. (n = 44/144) and Shigella spp. (n = 11/144): (a) The number of positive Salmonella samples; (b) The number of postitive Shigella samples of pooled oysters and estuarine water from Phang Nga Bay, southern Thailand. Sample collection started from March 2016 (month 1) to February 2017 (month 12).

Figure 3

Heavy metal concentration (ppm) of manganese (Mn), cadmium (Cd), and lead (Pb): (a) oyster meat samples; (b) estuarine waters from Phang Nga Bay, southern Thailand. Samples were collected from March 2016 (month 1) to February 2017 (month 12).

Figure 3

Heavy metal concentration (ppm) of manganese (Mn), cadmium (Cd), and lead (Pb): (a) oyster meat samples; (b) estuarine waters from Phang Nga Bay, southern Thailand. Samples were collected from March 2016 (month 1) to February 2017 (month 12).

Figure 4

Predicted average concentration of EC (MPN/g of oyster meat) as a function of: (a) average precipitation over 7 days stratified by the presence of Salmonella spp.; (b) temperature prior to sampling; (c) relative humidity (%). Note: (b,c) were predicted based on the presence of Salmonella in the oyster sample.

Figure 5

Predicted average concentration of EC (MPN/100 mL of estuarine water) as a function of: (a) average precipitation over 7 days; (b) ambient air temperature

Figure 6

Predicted probability of detecting Salmonella in pooled oyster samples as a function of: (a) average precipitation during previous 7 days; (b) the levels of EC (MPN/g oyster meat).

Figure 7

Accuracy for using EC to predict Salmonella contamination in pooled oyster samples: (a) ROC curve; (b) sensitivity and specificity for using EC concentration (MPN/g oyster meat) to predict Salmonella contamination in oysters, with a vertical line indicating the optimal cutoff value of 420 EC (MPN/g) based on the Youden index.