Phlegmy Bacteria Upon Coral - the importance of cross species interactions

Corals are holobionts; which is a symbiotic relationship between an invertebrate animal, a dinoflagellate and the dinoflagellate’s associated bacteria, and the dynamic between them determines the health of a reef. Differences in the microbial communities can be seen between healthy and diseased reefs, suggesting bacteria play a crucial role in the resistance to diseases. This paper focuses on understanding the interactions between native microbiota and known pathenogenic bacteria.

First they showed that small molecular weight compounds extracted from coral mucus inhibited the growth of ɣ-proteobacteria Serratia marcescens (a necrotizing coral pathogen). They then screened 300 different species of marine bacteria, all of which can be found on mucus of the elkhorn coral, to see if they could stimulate or inhibit ‘Quorum Sensing (QS) reporters’, and ended with 13 isolates from the genera; Photobacterium, Marinobacter, Vibrio, Caryophanon, Planomicrobium and α-proteobacteria which could all produce Acyl-Homoserine-Lactones (AHL’s being QS molecules).

They plated colonies of these bacteria next to colonies of 3 strains of S.marcescens; a wild-type, a strain isolated from a white pox disease lesion on a coral, and a mutant strain of S.marcescens which could only swarm if the bacterial colonies produced AHLs. This was done to see the effect on multicellular surface motility or ‘swarming’, which is known to be controlled by the QS-mediated production of surfactants. Five colonies inhibited the wild-type and white-pox isolates, a further five only inhibiting the wild-type. Vibrio spp. and Marinobacter spp. colonies restored the swarming in the mutant strain suggesting although the other bacterial colonies are manufacturing AHLs, they are also producing yet-unknown chemicals which inhibit swarming.

The relationship between swarming and biofilm production within populations of ɣ-proteobacteria have already been shown to be inversely regulated, so Alagely et al. (2011) tested the hypothesis: bacterial colonies known to stimulate swarming in ɣ-proteobacteria should inhibit the formation of a biofilm. The white-pox-isolate strain of S.marcescen was used as only 5 bacterial colonies had been shown to inhibit swarming, providing a good comparison to the bacteria which do inhibit swarming. The bacteria which provided the largest inhibition of biofilm were indeed shown to be the colonies which stimulated swarming in the earlier experiment, and conversely one bacterial colony which inhibited swarming promoted the formation of a biofilm. However five colonies tested inhibited both swarming and biofilm production. Finally they showed that coral polyps coated with either α-proteobacteria or a mixture of Marinobacter aided in the recovery of the coral after infection with the white-pox S.marcescen.

I chose this paper as I do think the authors showed evidence of a relationship between bacteria, most of which were capable of inducing a behavioural change in the ɣ-proteobacteria. They also show the importance in future studies of coral health to appreciate the dynamic relationship that the bacteria provide within the holobiont. This paper is novel in the approach to coral health by focusing specifically on the relationships between bacterial populations, whereas previous studies focus on how the invertebrate can control bacteria through the mucosal layer.

Signaling-mediated cross-talk modulates swarming and biofilm formation in a coral pathogen Serratia marcescens. Ali Alagely, Cory J Krediet, Kim B Ritchie and Max Teplitski. (2011)