Classification and spoilage characteristics of Chryseobacterium isolates from fish
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Date
2019
Authors
Gavu, Lydia
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Publisher
University of the Free State
Abstract
The genus Chryseobacterium was initially reported to have only six species and the number has increased rapidly over the years. The genus is represented by a total of 113 species at present. Species belonging to the genus Chryseobacterium are widespread in nature. They have been reported from clinical, environmental, industrial and food sources. Their spoilage characteristics have been well defined in food products including poultry, meat, milk and milk products and fish. Development of unpleasant odours in food contaminated by Chryseobacterium species usually results from the activity of proteolytic enzymes. The first aim of this study was to classify 11 potential Chryseobacterium fish isolates from a previous study. The methods that were used included 16S rRNA gene sequence analysis, conventional phenotypic methods and the BIOLOGTM Omnilog identification system. Chryseobacterium balustinum, C. gleum and C. piscium were used as reference strains throughout the whole study. Phylogenetic analysis based on 16S rRNA gene sequences indicated that the 11 Chryseobacterium isolates represented members of the genus Chryseobacterium. The highest similarity were obtained for C. piscium by strains SH 23-4 (98.89%), SH 28-3 (100%) and SH 30-1 (100%) and it was therefore concluded that these three strains could be other strains of C. piscium. This finding was confirmed by the phylogenetic treeing methods used in this study because the three strains clustered closely with C. piscium. Strains SH 11-3(a), SH 11-3(b), SH 20-4, SH 25-4, SH 40-3, SH 19-2(b) and SH 11-4(b) had sequence similarity values lower than 98.7%, it is therefore a possibility that these strains might be novel species of Chryseobacterium, however, further investigations need to be performed for confirmation. The BIOLOGTM Omnilog Gen III identification system and conventional phenotypic methods were also useful in classification. The BIOLOGTM Omnilog was also able to identify strains SH 23-4, SH 28-3 and SH 30-1 as possible strains of C. piscium. The other strains could not be identified because of its limited database. The second aim of this study was to estimate the spoilage potential of the 11 Chryseobacterium isolates by determining their spoilage potential in terms of substrate oxidation by the BIOLOGTM Omnilog system. The results indicated that all the isolates could be potential fish spoilers, however, strains SH 11-4(b) and SH 23-4 were able to oxidise the most of the carbon sources (25/31) and could be regarded as being able to cause the most spoilage. Sensory analysis of inoculated FJB samples did not show significant differences in terms of odour production, however, odours such as smelly feet, cabbage-like, fruity, sour and putrid sewage were noted for the 11 Chryseobacterium isolates and their reference strains. Chryseobacterium species were able to produce a total of 87 volatile compounds in fish juice broth (FJB) at 4 and 25 °C. The composition of the volatile compounds detected was, however, slightly higher for samples that were incubated at 25 °C than at 4 °C. The frequently detected compounds occured in FJB samples containing C. gleum and strain SH 30-1 and the least detected compounds occurred in FJB samples containing strains SH 19-2(b) and SH 11-4(b). The frequently produced compounds with more pronounced odours were identified as 2-ethyl-1-hexanol, indole, dimethyl disulphide and 2-phenylethanol. FJB samples inoculated with strains SH 30-1, SH 28-3, SH 25-4 had the majority of volatile compounds associated with unpleasant odours, such as fishy from trimethylamine; spoiled, putrid from dimethyl sulphide; and faecal, nauseating from indole. It was concluded that 7/11 Chryseobacterium strains in this study could be novel Chryseobacterium species. Sensory analysis and Gas-Chromatrography/Mass-Spectrometry were useful in estimating the spoilage potential of the Chryseobacterium species through odour production. The BIOLOGTM system can be used as an effective screening method for identifying the carbon sources utilised by the Chryseobacterium species which could then be investigated further for the potential of these starins to cause food spoilage. Chryseobacerium species used in this study have the potential to spoil fish and/other fish products because of their psychrotolerant and proteolytic nature.
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Keywords
Chryseobacterium, Taxonomy, Fish spoilage, Sensory analysis, Volatile production, Dissertation (M.Sc. (Food Microbiology))--University of the Free State, 2019