Relationships between environmental factors and pathogenic Vibrios in the Northern Gulf of Mexico

Abstract

Although autochthonous vibrio densities are known to be influenced by water temperature and salinity, little is understood about other environmental factors associated with their abundance and distribution. Densities of culturable Vibrio vulnificus containing vvh (V. vulnificus hemolysin gene) and V. parahaemolyticus containing tlh (thermolabile hemolysin gene, ubiquitous in V. parahaemolyticus), tdh (thermostable direct hemolysin gene, V. parahaemolyticus pathogenicity factor), and trh (tdh-related hemolysin gene, V. parahaemolyticus pathogenicity factor) were measured in coastal waters of Mississippi and Alabama. Over a 19-month sampling period, vibrio densities in water, oysters, and sediment varied significantly with sea surface temperature (SST). On average, tdh-to-tlh ratios were significantly higher than trh-to-tlh ratios in water and oysters but not in sediment. Although tlh densities were lower than vvh densities in water and in oysters, the opposite was true in sediment. Regression analysis indicated that SST had a significant association with vvh and tlh densities in water and oysters, while salinity was significantly related to vibrio densities in the water column. Chlorophyll a levels in the water were correlated significantly with vvh in sediment and oysters and with pathogenic V. parahaemolyticus (tdh and trh) in the water column. Furthermore, turbidity was a significant predictor of V. parahaemolyticus density in all sample types (water, oyster, and sediment), and its role in predicting the risk of V. parahaemolyticus illness may be more important than previously realized. This study identified (i) culturable vibrios in winter sediment samples, (ii) niche-based differences in the abundance of vibrios, and (iii) predictive signatures resulting from correlations between environmental parameters and vibrio densities.

FIG. 1.

Densities of V. parahaemolyticus and V. vulnificus, as quantified by DP/CH for tlh and vvh markers, respectively, over the course of the 19-month study. Red squares, density of tlh or vvh in number of CFU/ml (water) or CFU/g (oysters and sediment); blue diamonds, sea surface temperature in °C. Red arrow identifies nondetects, in which the number of CFU were below the limits of detection by DP/CH analysis. For graphing purposes, nondetects (zeros) were replaced with 20% of the limits of detection, which were 0.2 CFU/ml water, 2 CFU/g oyster, and 4 (V. parahaemolyticus) or 20 (V. vulnificus) CFU/g sediment. Note that V. parahaemolyticus and V. vulnificus remain culturable from sediment, even as temperatures drop below 15°C.

FIG. 2.

Relative abundance of tlh compared to vvh (tlh-to-vvh ratio) during the 19-month sampling period in water (diamonds), oysters (triangles), and sediment (squares). Temperature is indicated by the connected black squares. Median ratios for water, oysters, and sediment (red lines) were 0.5, 0.7, and 3.5, respectively. Ratios were calculated as tlh densities (number of CFU/g or ml) divided by vvh densities (number of CFU/g or ml); thus, values of greater than 1.00 indicate that the tlh density was higher than the vvh density in that particular sample, while values of less than 1.00 indicate the opposite.

FIG. 3.

Partial coefficients of determination (Pseudo-R²) for temperature (Temp., black), salinity (Sal, diamonds), log₁₀ chlorophyll a (Chl-a, stripes), and log₁₀ turbidity (Turb., gray) (A) and regression coefficients for temperature, salinity, quadratic salinity (Salinity²), log₁₀ chlorophyll a (Chl-a), and log₁₀ turbidity (B). Densities of tlh, tdh, trh, and vvh were measured in water (wat), oysters (oys), and sediment (sed); only statistically significant partial coefficients of determination and regression coefficients (P < 0.05) are presented.