Another threat to coral reefs, are human pathogen’s causing coral diseases?

Acropora palmata was once one of the most abundant corals found in the Caribbean; however in 2006 it was listed under the United State endangered species act. The cause of the decline in coral numbers is mostly due to a disease that is unique to this coral species, acroporid serratiosis (APS), or more commonly known was white pox. The bacterium which causes this disease is Serratia marcescens, it is also known to cause respiratory and urinary tract infections, meningitis and pneumonia in humans. Following a major outbreak of the disease in corals in 2003 it was found that the strain PDR60 of the bacterium was found in untreated human waste water and diseased A.palmata, this suggests that human sewage could be a cause of the increase in the prevalence of the coral disease.

Because of the endangered status of A.palmata only 8 treatments and 3 controls could be performed, healthy corals not infected with APS were collected and inoculated S.marcescens of different strain and origin, strains were determined using a three step method, culturing on MCSA then on DNase with Toluidine Blue agar and then PCR.

1. Strain PDR60 from APS affects A.palmata

2. Strain PDR60 from APS affects A.palmata

3. Strain PDR60 from APS affects A.palmata

4. Strain PDR60 from an apparently healthy non host coral Siderastrea siderea

5. Strain PDR60 from the snail Coralliophila abbreviata

6. Strain PDR60 from wastewater

7. Strain PDL100 ( confirmed APS pathogen) APS affected A.palmata

8. Strain ww131 from wastewater

Controls include : E.Coli, sterile CaCO₃ sediment (vehicle control) and seawater.

After inoculation the number of days taken until tissue loss occurred was measured, once tissue loss occurred the surface mucopolysaccharide layer (SML) was collected using a needle, the SML contained necrotic snail tissue. The SML was centrifuged and then spread onto MCSA agar, positive bacterial colonies appear red/pink which is characteristic of a S.marcescens culture. The positive bacterial cultures were plated onto DTC plates, DTC presumably stands for the type of agar however this is not explained. If the DTC plates were positive for S.marcescens, this appears as a red halo on the DTC plate they were then plated onto non selective TS agar to form pure cultures.

Isolates from APS affected coral, which after growth on MSCA and DTC plates were presumed to be S.marcescens, underwent a Seratia-specific PCR, band pattern were used to assess the relatedness between strands. This allowed the confirmation that the bacteria collected from the APS legions was in fact the same strain that the coral fragments had been inoculated with. This method of proving disease is known as Koch’s postulates, Koch’s postulates has identified strain PDR60 in host and non-host corals and also snails, the aim of the paper was to assess whether the strains from the non-host coral S.sidera and the snail C.abbreviata along with wastewater strains could cause disease in a healthy A.palmata.

Strain PDR60 from wastewater and diseased A.palmata caused disease within four and five days respectively, this shows that wastewater is a source of APS, furthermore the human strain was shown to be a coral pathogen through Koch’s postulates.

The inoculates were considered virulent if tissue loss occurred in the corals before day 15, as this was when the control vehicle (sediment) isolate caused tissue loss. No S.marcescens was detected in the water or in the coral fragments before exposure. The strains collected from C.abbreviata and S.sidera caused disease after the 15 days period observed in the control (CaCO₃ sediment).Therefore it is possible another stressor may have caused the tissue loss, or however the strains isolated from non-host coral and snails could in fact be virulent but didn’t show signs because the A.palmata fragments may have been resistant to APS. This hypothesis is supported by the fact that the isolate 3 which was extracted from APS affected coral did not cause disease in the corals which were exposed to it. Strain PDL100 of the bacterium S.marcescens had limited pathogenicity and the pathogenicity has been shown to of decreased in recent years. Along with V.shiloi , a bacteria which causes coral bleaching, and A.corallicida, which causes white plaque. The author states that this may be because of development of host resistance to the bacterial strains. Examples of further studies would need to assess the susceptibility of host corals and also the genetic variation between S.marcescens strains and isolates.

The importance of understanding that bacterial strains from human wastewater can cause disease in coral is high; the majority of the sewage from around the Florida Keys and the Caribbean is not treated and is disposed of within a limestone substrate that allows leaking of sewage into coastal waters. Coral reefs are already threated in a variety of different ways, this study outlines the need for waste water disposal reforms as advanced wastewater treatments can reduce S.marcescens to undetectable levels. The transfer of a human disease to another animal is known as reverse zoonosis, however in this case the pathogen has passed from a vertebrate to an invertebrate and from terrestrial environment to a marine one. Further research into the tolerance of the human strain to salinity may be important as if this strain is able to tolerate high salinity levels it may become prevalent on reefs causing reoccurring infections.