Impacts of Hurricanes Katrina and Rita on the microbial landscape of the New Orleans area

Abstract

Floodwaters in New Orleans from Hurricanes Katrina and Rita were observed to contain high levels of fecal indicator bacteria and microbial pathogens, generating concern about long-term impacts of these floodwaters on the sediment and water quality of the New Orleans area and Lake Pontchartrain. We show here that fecal indicator microbe concentrations in offshore waters from Lake Pontchartrain returned to prehurricane concentrations within 2 months of the flooding induced by these hurricanes. Vibrio and Legionella species within the lake were more abundant in samples collected shortly after the floodwaters had receded compared with samples taken within the subsequent 3 months; no evidence of a long-term hurricane-induced algal bloom was observed. Giardia and Cryptosporidium were detected in canal waters. Elevated levels of fecal indicator bacteria observed in sediment could not be solely attributed to impacts from floodwaters, as both flooded and nonflooded areas exhibited elevated levels of fecal indicator bacteria. Evidence from measurements of Bifidobacterium and bacterial diversity analysis suggest that the fecal indicator bacteria observed in the sediment were from human fecal sources. Epidemiologic studies are highly recommended to evaluate the human health effects of the sediments deposited by the floodwaters.

Fig. 1.

Floodwater depths and sampling sites. Color shadings indicate depth of floodwater (in meters) in NO as of August 30, 2005. Droplet symbols indicate the location of sites where only water samples were collected including LP water sample sites and canal sites. Circles indicate the location of sites where sediment samples were collected. For sites adjacent to canals (R1–R6), water samples were also collected. The cross shows the location of the 17th Street Canal Pump Station 6, the main discharge point for the inner city canals into LP.

Fig. 2.

Chlorophyll-a concentrations in LP, within the 17th Street and Industrial Canal of NO, in Chandeleur Sound–Mississippi River Delta area, and in the Gulf of Mexico after Hurricane Katrina. Samples were collected during two cruises (September 13–26 and 22–25, 2006) sponsored by National Oceanic and Atmospheric Administration. Data shown in Inset for Lake Pontchartrain were collected on November 2, 2005.

Fig. 3.

Fecal indicators, enterococci and E. coli, in LP transect B and canal sites including results from Bacteroidales analysis by PCR and Bifidobacterium. Results for the fecal indicators correspond to analysis using MF, CS, and qPCR. Error bars correspond to 95% confidence limits (Student's t test). Data points shown with error bars correspond to the average for samples collected between October 11 and November 12, 2005. For lake transects, the samples were also averaged over transects A, B, and C. Site S1 was sampled both before and after pumping at the 17th Street Canal Pump Station 6. For Bacteroidales and Bifidobacterium the number in the numerator corresponds to the number of positive samples and the number in the denominator corresponds to the total number of samples analyzed. “na” indicates not analyzed. See SI Tables 2 and 3.

Fig. 4.

Comparative bacterial community analysis of samples collected from the 17th Street Canal on two separate occasions, from the Industrial Canal and a residential yard. Analyses are based on sequencing of random clones from libraries of PCR-amplified small-subunit rRNA gene fragments with sequences being binned into habitat types based on similarity to top scoring BLAST hits.

Fig. 5.

Abundance of Vibrio species in LP transects and canal sites during the October 11, 2005, and January 27, 2006, sampling dates. Total Vibrio spp. abundance is based on total colony-forming units on TCBS medium. Abundances of V. cholerae, V. vulnificus, and V. parahaemolyticus are estimated from the proportion of total colony-forming units testing positive for each of these species by biochemical and genetic tests. “Other” refers to those colonies not classified as one of the three targeted pathogens. Data shown are representative of the data collected. For lake transects, the samples were also averaged over three available transects. See SI Table 4.

Fig. 6.

Abundance of enterococci in sediments of the NO area, including results from Bifidobacterium analysis for samples collected March 25, 2006. (+, positive; −, not detected). Enterococci results correspond to samples collected between November 12, 2005, and June 20, 2006, and analyzed using the CS method. Error bars correspond to 95% confidence limits (Student's t test). See SI Table 5.