Abstract
Cholera has been endemic to the Ganges Delta for centuries. Although the causative agent, Vibrio cholerae, is autochthonous to coastal and brackish water, cholera occurs continually in Dhaka, the inland capital city of Bangladesh which is surrounded by fresh water. Despite the persistence of this problem, little is known about the environmental abundance and distribution of lineages of V. cholerae, the most important being the pandemic generating (PG) lineage consisting mostly of serogroup O1 strains. To understand spatial and temporal dynamics of PG lineage and other lineages belonging to the V. cholerae species in surface water in and around Dhaka City, we used qPCR and high-throughput amplicon sequencing. Seven different freshwater sites across Dhaka were investigated for six consecutive months, and physiochemical parameters were measured in situ. Total abundance of V. cholerae was found to be relatively stable throughout the 6-month sampling period, with 2 × 105 to 4 × 105genome copies/L at six sites and around 5 × 105genome copies/L at the site located in the most densely populated part of Dhaka City. PG O1 V. cholerae was present in high abundance during the entire sampling period and composed between 24 and 92% of the total V. cholerae population, only showing occasional but sudden reductions in abundance. In instances where PG O1 lost its dominance, other lineages underwent a rapid expansion while the size of the total V. cholerae population remained almost unchanged. Intraspecies richness of V. cholerae was positively correlated with salinity, conductivity, and total dissolved solids (TDS), while it was negatively correlated with dissolved oxygen (DO) concentration in water. Interestingly, negative correlation was observed specifically between PG O1 and salinity, even though the changes in this variable were minor (0-0.8 ppt). Observations in this study suggest that at the subspecies level, population composition of naturally occurring V. cholerae can be influenced by fluctuations in environmental factors, which can lead to altered competition dynamics among the lineages.
Highlights
Vibrio cholerae is a normal inhabitant of aquatic environments such as rivers, estuaries and coastal waters and has been detected in diverse geographic locations worldwide[1]
A 272 bp hypervariable stretch of the viuB marker gene, which is present in a single copy in V. cholerae [21] was amplified and sequenced from fortnightly samples taken in seven different water reservoirs in and around Dhaka city from October 2015 to Mach 2016 (Fig. 1)
This study revealed the consistent presence of pandemic generating (PG) V. cholerae lineage in the natural water bodies of Dhaka at a considerable proportion of the total V. cholerae population, suggesting that the pathogenic bacterium is circulating year-round in this urban aquatic environment, or is constantly shed by human carriers
Summary
Vibrio cholerae is a normal inhabitant of aquatic environments such as rivers, estuaries and coastal waters and has been detected in diverse geographic locations worldwide[1]. Million cases and 21, 000 to 143, 000 deaths worldwide each year [4]. Seven cholera pandemics have struck human civilization so far; the first pandemic of cholera started in 1817 and was followed by six others in the two hundred years, leaving a devastating human death toll [5]. V. cholerae is a diverse species, with more than 200 serogroups being identified based on their surface polysaccharide O antigen [6]. Only O1 serogroup dominates the pandemic generating (PG) lineage [7, 8], which has been responsible for all seven cholera pandemics, and other serogroups mostly represent V. cholerae environmental strains which are generally non-pathogenic [5, 9]. Serogroup O1 strains of the PG lineage are further classified into two biotypes, the Classical biotype that was shown to cause the fifth and sixth pandemic and believed to be associated with the earlier pandemics, and the El Tor biotype which is the causative agent for the seventh pandemic of cholera [5, 8]
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