Systematic Analysis of White Pox Disease in Acropora palmata of the Florida Keys and Role of Serratia marcescens

Abstract

White pox disease (WPD) affects the threatened elkhorn coral, Acropora palmata. Owing in part to the lack of a rapid and simple diagnostic test, there have been few systematic assessments of the prevalence of acroporid serratiosis (caused specifically by Serratia marcescens) versus general WPD signs. Six reefs in the Florida Keys were surveyed between 2011 and 2013 to determine the disease status of A. palmata and the prevalence of S. marcescens. WPD was noted at four of the six reefs, with WPD lesions found on 8 to 40% of the colonies surveyed. S. marcescens was detected in 26.9% (7/26) of the WPD lesions and in mucus from apparently healthy colonies both during and outside of disease events (9%; 18/201). S. marcescens was detected with greater frequency in A. palmata than in the overlying water column, regardless of disease status (P ‫؍‬ 0.0177). S. marcescens could not be cultured from A. palmata but was isolated from healthy colonies of other coral species and was identified as pathogenic pulsedfield gel electrophoresis type PDR60. WPD lesions were frequently observed on the reef, but unlike in prior outbreaks, no wholecolony death was observed. Pathogenic S. marcescens was circulating on the reef but did not appear to be the primary pathogen in these recent WPD episodes, suggesting that other pathogens or stressors may contribute to signs of WPD. Results highlight the critical importance of diagnostics in coral disease investigations, especially given that field manifestation of disease may be similar, regardless of the etiological agent.T he combination of physical stress and disease has resulted in the decline of corals and coral reefs throughout the Caribbean (1, 2). In the Florida Keys and elsewhere, the iconic elkhorn coral (Acropora palmata) has experienced precipitous declines due in part to white pox disease (WPD) (3, 4). Following outbreaks of WPD in the late 1990s and early 2000s, the bacterium Serratia marcescens was isolated from diseased corals and subsequently identified as an etiological agent by fulfillment of Koch's postulates (4, 5). As originally proposed (4), to distinguish the disease caused by this bacterium from broader signs of WPD, it is referred to as acroporid serratiosis when, and only when, S. marcescens is confirmed from a lesion on an A. palmata colony.Between 1999 and 2006, two strains of S. marcescens were associated with large outbreaks of WPD (acroporid serratiosis) in the Florida Keys. The strain found in association with outbreaks in 2002 and 2003 was identical to a strain concurrently found in human sewage from the nearby islands that compose the Florida Keys archipelago (5, 6). Until recently, septic systems and cesspits were the primary mechanism of wastewater disposal (7-9). Inground disposal of waste led to sewage pollution in both nearshore and offshore waters of the Florida Keys (10-12).In recent studies in the Florida Keys and elsewhere in the Caribbean, researchers have reported that S. marcescens could not always be isolated from colonies displaying signs of WP...

Shifting white pox aetiologies affecting Acropora palmata in the Florida Keys, 1994–2014

“…Finally, acroporid serratiosis was first diagnosed in 1999 with the isolation of S. marcescens from A. palmata with WPX signs. This form of WPX has been found repeatedly [65] and as recently as 2013 [34], but the absence of S. marcescens from some corals exhibiting WPX signs [34,65,72] suggests that other forms of WPX exist [34,65]. Lack of consistent isolation of established pathogens suggests that the same disease signs can be caused by more than one aetiologic agent and supports the moving target hypothesis that historical and contemporary states of a disease differ.…”

“…An established WPX pathogen, S. marcescens, was cultured from lesions during historical outbreaks in 1999 and 2003, confirming acroporid serratiosis for these cases of WPX [9,33] (table 1) (table 2) [34]. Serratia marcescens was not detected from WPX lesions with culture methods.…”

“…A form of WPX called acroporid serratiosis is diagnosed if and only if classic signs of WPX affecting the A. palmata host co-occur with the presence of the established bacterial pathogen Serratia marcescens [9] (table 1). Other forms of WPX with the same gross signs on the same host species but not associated with S. marcescens also exist [34,65]. Aetiology of the alternative form(s) is unknown.…”

Antimicrobial and stress responses to increased temperature and bacterial pathogen challenge in the holobiont of a reef‐building coral

“…Similarly, Vibrio shilonii used to be implicated in bleaching in the coral Oculina patagonica (Kushmaro, Loya, Fine, & Rosenberg, ; Kushmaro, Rosenberg, Fine, & Loya, ); however, following several outbreaks, infection by this pathogen has not been observed in the field and cannot be re‐established experimentally (Reshef, Koren, Loya, Zilber‐Rosenberg, & Rosenberg, ). Serratia marcescens has also been reported to be incapable of infecting Acropora palmata , despite being associated with white pox in this species a decade earlier (Joyner et al., ). While the exact mechanisms of resistance to pathogens are unknown, it could be the result of the selective elimination of disease‐sensitive corals from the population, holobiont adaptation or potentially immunological memory.…”

Complementary approaches to diagnosing marine diseases: a union of the modern and the classic

“…Source tracking investigations, combining classic culture and modern molecular techniques, identified human wastewater as a source of S. marcescens [88,89] contributing to initiation of upgrades (in-ground-waste to central sewer systems with at least secondary treatment) in sewage treatment Florida Key-wide, with a completion date in late 2015 [90]. qPCR has since been developed to more rapidly detect the S. marcescens pathogen from SMLs of affected hosts [91,92].…”