Persistent Vibrios – evidence why they won’t leave us alone

Gram-negative marine bacteria, pathogenic Vibrio species, are common and abundant in estuaries and coastal habitats. Species densities correlate with environmental seasonality. Increasing during spring and summer months when temperatures are 20-30°C and salinities 10-20ppt (DePaola et al., 2003), as well as occurring more consistently within harmful algal blooms of Ostreopsis (Bellés-Garulera et al., 2016). During these periods, an increased risk of human infection occurs both directly and indirectly by V. parahaemolyticus and V. vulnificus which cause gastroenteritis and septicaemia by foodborne pathogens in shellfish. These bacteria are found in sediments, plankton and water and its recovery during the seasonal re-emergence in shellfish is of great interest in health and aquaculture.

Givens et al., (2014) collected 10 common estuarine fish species and 6 oysters, along with sediment and water samples, from Dauphin Island Bay, Albama at 12 points during March 17-May 02, 2011. Measuring water temperature and salinity using a YSI 85 metre. The contents of the mid- to hind-gut regions of the fish digestive tracts were pooled and a control of alkaline peptone water culture of V. cholera was added overnight, quantifying ctx gene recovery using qPCR, a culture-independent method. Various DNA extraction kits were used for; 1.0g of fish gut substrate, 0.5g of pooled, blended Oyster meats and liquids and 100ml water sample filtered through a 0.22µmol l-1 nitrocellulose filter. Secondly, the culture-dependent method, 100µl of each homogenised sample was spread-plated on V. vulnificus and T₁N₃ agars, with addition of 0.1g homogenised Oyster and 10ml of water spread-plated onto VVA and T₁N₃ agars, 35ºC incubated overnight. Developed bacterial colonies were hybridized using DNA probes targeting V. vulnificus vvhA and V. parahaemolyticus tlh genes, using 16 rRNA in PCR. Virulence potential was gained by spread-plating on agar plates. Quantified in PCR but found low virulence genotypes. Their novel methods were detailed, however, it would have been useful if pH, nutrients and trace metals were also included to see if these factors had an influence on the densities as well.

Results of qPCR found highest densities of V. vulnificus than V. parahaemolyticus within the fish intestines when co-occurring. Overall, Bacteria abundance is 1.7ml^-1 greater than Oysters, 0.6ml^-1 than sediment and 3.2ml^-1 than water, with Vibrios being 3.7ml^-1 greater than Oyster, 4.2ml^-1 than sediment and 5.9ml^-1 than water. The authors results contradict and back up previous studies. They found similar sample density results to DePaola et al., (1994), however, Jones et al, (2013) detected higher densities for V. parahaemolyticus than V. vulnificus in fish intestines. All three studies varied in season of samples taken, thus, temperature influences Vibrio species densities occurring. Vibrios correlate to environmental conditions in this study. Water temperature ranged from 20-25ºC and salinity from 3.7-19ppt therefore, higher densities as its more favourable environments. The low salinity, cooler temperature samples were less favourable for V. vulnificus, qPCR could not detect the low abundances, and V. parahaemolyticus only detected in fish. Therefore, a higher resolution culture-dependent-method (colony hybridisations) was used, resulting in both species detected but, at lower densities. The author suggests this higher density prevalence in the intestine niche during the limiting conditions, causes the re-emergence of the pathogenic Vibrios in Oysters.

This interested me as Vibrios affect a broad range of habitats and species and climate change could intensify its persistence and dominance in more species and areas than found today. A novel study as it is the first to conduct and compare two culture methods, by comparing results were more defined as each method was limiting in some way, but similar trends were seen. Future study for Vibrios in different species has future importance as it defines not just environmental factors causing Vibrio species to bloom, but also in which species it may keep re-occurring. This narrows the knowledge gap, leading to further infection prevention, helping decrease coastal human infections by sectioning off habitats and aquaculture prevention.

Reviewed Article

Givens, C. E., Bowers, J. C., DePaola, A., Hollibaugh, J. T., & Jones, J. L. (2014). Occurrence and distribution of Vibrio vulnificus and Vibrio parahaemolyticus – potential roles for fish, oyster, sediment and water. Letters in Applied Microbiology, 58, 503-510 http://onlinelibrary.wiley.com/doi/10.1111/lam.12226/full

Further Reading

Bellés-Garulera, J., Vila, M., Borrull, E., Riobó, P., Franco, J. M., & Sala, M. M. (2016). Variability of planktonic and epiphytic Vibrios in a coastal environment affected by Ostreopsis blooms. Scientia Marina, 80(S1) http://scientiamarina.revistas.csic.es/index.php/scientiamarina/article/viewArticle/1661/2144

DePaola, A., Nordstrom, J. L., Bowers, J. C., Wells, J. G., & Cook, D. W. (2003). Seasonal Abundance of Total and Pathogenic Vibrio parahaemolyticus in Albama Oysters. Applied and Environmental Microbiology, 69(3), 1521-1526                         http://aem.asm.org/content/69/3/1521.full