Thesis (MSc)--University of Stellenbosch, 2006. / ENGLISH ABSTRACT: Fish infections caused by pathogenic Flavobacterium spp. are a major problem in the aquaculture industry worldwide, often leading to large economic losses. Thirty-two Flavobacterium spp. isolates, obtained from various diseased fish species and biofilm growth, were characterised genetically using 16S rRNA gene sequencing, 16S rRNA gene PCR restriction fragment length polymorphism (RFLP), randomly amplified polymorphic DNA (RAPD) PCR, repetitive extragenic palindromic (REP) element PCR, plasmid profiling, whole cell protein (WCP) and outer membrane protein (OMP) analyses. The biofilm-forming capability of five genetically heterogeneous Flavobacterium spp. study isolates was investigated using a modified microtiter-plate adherence assay, as well as flow cell studies. Experimental infection studies with Mozambique tilapia (Oreochromis mossambicus) were carried out in order to determine the virulence of the Flavobacterium spp. study isolates. 16S rRNA gene sequence analysis showed the Flavobacterium spp. study isolates were closely related, and 97% sequence similarity was shared with published F. johnsoniae sequences. A high degree of genetic heterogeneity was displayed by the Flavobacterium spp. study isolates following RAPD-PCR, REP-PCR and OMP analysis, however, based on the results obtained by plasmid profiling and WCP analysis, the isolates appeared genetically very homogeneous. The biofilm phenotype was displayed by all five Flavobacterium spp. isolates tested and varied from weakly to strongly adherent. No specific correlation was observed between the RAPD, REP and/or OMP profiles and degree of adherence displayed by Flavobacterium spp. isolates. However, a specific WCP profile (profile B), exhibited by 48% of the Flavobacterium spp. isolates, was linked to strong adherence. Experimental infection studies showed that Flavobacterium spp. isolates displayed variable levels of virulence, which could not be linked to biofilm formation, nor specific genotypes. This is the first reported isolation and characterisation of Flavobacterium spp. isolated from diseased fish in Southern Africa, and there appears to be significant diversity amongst the isolates which is not geographically linked nor host related. / AFRIKAANSE OPSOMMING: Visinfeksies veroorsaak deur Flavobacterium spp. is problematies in die akwakultuur industrie wêreldwyd en lei tot groot ekonomiese verliese. Twee en dertig Flavobacterium spp. isolate, geïsoleer vanaf verskye geïnfekteerde visspesies en biofilm groei, was geneties gekarakteriseer met behulp van 16S rRNS geenvolgorde, 16S rRNS geen PKR restriksie fragment lengte polimorfisme (RFLP), toevallig geamplifiseerde polimorfiese DNS (TGPD) PKR, herhaalde ekstrageniese palindromiese (HEP) element PKR, plasmied profilering, heelsel protein (HSP) en buite membraan protein (BMP) analise. Die vermoë van vyf geneties heterogene Flavobacterium spp. isolate om biofilms te vorm was ondersoek met behulp van ‘n gemodifiseerde mikrotiterplaat vashegtings toets asook vloei-sel studies. Eksperimentele infeksie studies was uitgevoer op bloukurpers (Oreochromis mossambicus) om die virulensie van die Flavobacterium spp. studie isolate te toets. 16S rRNS geenvolgorde analise het getoon dat die Flavobacterium spp. studie isolate naby verwant was, en het 97% ooreenstemming getoon met gepubliseerde F. johnsoniae volgordes. TGPD-PKR, HEP-PKR en BMP analise het ‘n hoë graad van heterogeniteit tussen die Flavobacterium spp. studie isolate aangetoon, egter, op grond van plasmied profilering en HSP analise, was die studie isolate geneties baie homogeen. Die biofilm fenotipe was getoon deur al die getoetsde Flavobacterium spp. isolate en het gevarieer van swak tot sterk vashegting. Geen spesifieke korrelasie was waargeneem tussen die TGPD, HEP en/of BMP profiele en graad van vashegting vertoon deur Flavobacterium spp. isolate nie, maar ‘n spesifieke HSP profiel (profiel B), getoon deur 48% van die Flavobacterium spp. isolate, was verbind met sterk vashegting. Eksperimentele infeksie studies het getoon dat Flavobacterium spp. isolate varierende grade van virulensie vertoon het en wat met biofilm formasie of spesifieke genotipes geassosieer kon word nie. Hierdie is die eerste gedokumenteerde isolasie en karakterisering van Flavobacterium spp. geïsoleer van geïnfekteerde vis in Suider Afrika, en daar is beduidende diversiteit tussen die isolate wat nie geografies of gasheer geassosieerd is nie.