Studies on the virulence of Aerococcus viridans (var.) homari, the causative agent of gaffkemia, a fatal disease of homarid lobsters

Abstract

Virulent and avirulent strains of Aerococcus viridans (var.) homari were used to extend previous studies to determine and confirm differences between the 2 types. Virulent strains possessed polysaccharide capsules and were not agglutinated by lobster hemolymph serum; avirulent strains did not have capsules, were agglutinated by the lobster hemolymph serum, and most did not grow well in lobster hemolymph serum. Growth of the avirulent strains in sterile lobster hemolymph serum induced the production of capsules (which reached a maximum after 5 to 7 d incubation), eliminated susceptibility of the strains to the lobster serum agglutinin, and restored their virulence against lobsters. The factor(s) in lobster hemolymph serum inducing the long-lasting phenotypic response of virulence was (were) heat labile. KEY WORDS: Lobster · Homarus americanus · Virulence · Aerococcus viridans (var.) homari · Capsules · Agglutinin Resale or republication not permitted without written consent of the publisherDis Aquat Org 60: [149][150][151][152][153][154][155] 2004 nated by the lobster serum agglutinins, while avirulent strains were. Additionally, 24 h growth trials showed that virulent strains formed substantial capsules when grown in lobster hemolymph serum, while under the same time and conditions avirulent strains produced only trace amounts of capsular materials.In this paper we report extensions of these investigations conducted to determine the degrees of variation in virulence among different strains of Aerococcus viridans (var.) homari, the coincidence of capsular materials with virulence, the stability of virulence and its restoration. MATERIALS AND METHODSLobsters. Freshly captured lobsters of both sexes (weighing between 450 and 500 g) were obtained directly from fishermen in Eastern Passage and Digby, Nova Scotia. The chelae were immobilized with heavy rubber bands and the lobsters were held in tanks supplied with running, filtered seawater at ambient temperatures ranging from 1 to 12°C, depending upon season. All experimental work was carried out in tanks of re-circulating, filtered seawater at 15 ± 0.5°C after the lobsters had been acclimated to this temperature for ca. 2 wk; the lobsters were not fed during the actual challenge trials. Individual shelters were provided in tanks to protect the lobsters during holding periods and experimental work. Prior to the initiation of experiments, all lobsters involved were examined by culture methods and hemolymph smears to ensure that they were not infected with Aerococcus viridans (var.) homari.Strains of Aerococcus viridans (var.) homari. Strains were isolated during various surveys and identified as described by Stewart et al. (1966), or purchased from the American Type Culture Collection (ATCC 10400); 1 strain of proven virulence, which we have used for reference purposes throughout much of the work on this disease, was donated by Dr. H. Rabin (hereafter referred to as Rabin's strain; Cornick & Stewart 1968). The virulence of Rabin's strain was maintained b...

Genetic characterization of the lobster pathogen Aerococcus viridans var. homari by 16S rRNA gene sequence and RAPD

“…Similar scenarios have been implicated in the spread of fish diseases and for the appearance of pathogens with the same DNA fingerprint pattern in different countries (Hänninen & Hirvelä-Koski 1999, Ravelo et al 2003. Stewart et al (2004) have recently shown that virulence in Aerococcus viridans var. homari can be induced by culturing avirulent strains in the presence of sterile lobster hemolymph serum.…”

“…homari strains in which no genetic variation was observed between the avirulent and virulent members of subtype 1. However, in the context of the study of Stewart et al (2004) Previous work on the development of an indirect fluorescent antibody technique (IFAT) for detection of A. viridans var. homari documented differences in IFAT staining between virulent (Rabin, NVI 1030, NVI 1032) and avirulent strains (ATCC 10400, 37R and 88B) that were correlated with variation in capsule thickness (Marks et al 1992).…”

“…Nevertheless, virulent strains of A. viridans var. homari maintained in vitro lose the capacity to cause disease in lobsters upon re-infection (Stewart 1984, Stewart et al 2004. Although the exact mechanisms of virulence and its subsequent loss through continuous passage in vitro are unknown, it appears to be correlated with the presence and thickness of the polysaccharide capsule (Stewart 1984, Stewart et al 2004.…”

“…On rare occasions, avirulent Aerococcus viridanslike cocci have been isolated from lobsters (Wood 1965b, Stewart et al 1966, Rabin & Hughes 1968, Stewart et al 2004). These cocci share many phenotypic characteristics with virulent A. viridans var.…”

Immunogen-dependent quantitative and qualitative differences in phagocytic responses of the circulating hemocytes of the lobster Homarus americanus

“…Virulent strains of Aerococcus viridans (var.) homari tend to lose their capsules and their virulence gradually with prolonged culture in the standard laboratory medium trypticase soy broth; the capsule and accompanying virulence have been restored in avirulent strains by growing them in lobster hemolymph serum (Stewart et al 2004b). Bacterins prepared using this pathogen have induced in lobsters different degrees of resistance which appear to vary as a function of the treatment applied to the immunogen(s) (Stewart & Zwicker 1974, Rittenburg 1981, Keith et al 1992, Stewart et al 2004a).…”