Streptococcus parauberis

This summer, I have had the chance to continue working with Stephen Cole on his master’s project regarding Streptococcus parauberis in cattle and fish strains.

Streptococcus parauberis is an important pathogenic bacterium known to cause two economically important diseases: mastitis in dairy cattle and streptococcosis in fish.  S. parauberis research has focused on the description of the bacterium and its outbreaks.  There has been limited research on targets for potential vaccines to combat these losses.  One way to study vaccine design is to target genes that are virulence factors — genes whose products increase disease potential.  One example of such virulence genes is the bacterial cell capsule. The capsule is an important protective feature of pathogenic bacteria because of its ability to protect against desiccation and prevent recognition by host immune defenses. Although capsules are usually composed of polysaccharides or polypeptides, the chemical composition varies between organisms.

The polysaccharide capsule of Streptococcus iniae, a close relative of S. parauberis, is well studied.  It is encoded by the cps operon which has been fully mapped and used as a target for live attenuated vaccine design (Locke et al. 2006).  It has been found to play a key role the virulence of S. iniae through its ability to lower the rate of phagocytosis by host immune cells.  In contrast, S. uberis, another close relative of S. parauberis, produces a hyaluronic acid (HAS) capsule encoded by up to three has genes.  It also is known to aid in resisting phagocytosis by neutrophils, an essential part of a mammal’s immune system (Ward et al. 2000).

Given that these close relatives produce different kinds of capsules, what kind of capsule does S. parauberis produce?  Previous work in Dr. Haines’ lab has identified both cps and has genes in S. parauberis. It remains unclear which genes are used and what type of capsule S. parauberis produces.  The project presented here aims to:

(1) finish mapping the cps operon and has genes present in S. parauberis

(2) study the expression of these genes in laboratory and experimental environmental conditions (i.e. in the presence of milk)

(3) determine the variability of capsule type and size based on expression data.

To understand the expression of these capsule genes, reverse transcriptase polymerase chain reaction (RT-PCR) will be performed. With this data, I will be able to determine which capsule genes S. parauberis uses:  has, cps, or a combination of both. I hypothesize that host type may determine the type of capsule present. For example, NCDO2020, isolated from cattle, will express a hyaluronic acid capsule, whereas T1, isolated from fish, will express a polysacchride capsule. This is based on the behavior of closely related species of S. uberis and S. iniae, respectively. If no differences are revealed when grown in highly nutrient laboratory media, I will alter growth media to mimic environments more representative of different host types to look at differential gene expression. For example, salinity levels, presence of lactose, and temperature can be tested. Once differences are identified, further studies using microscopy can be used to measure capsule size and reveal any differences between strains.

A research approach similar to this has been used by a group studying S. iniae at the University of California, Davis.  Their work led directly to the development of a live attenuated vaccine against that pathogen.  My hope is that this research can lead to an increased of the expression of virulence factors that may be responsible for the pathogenesis that leads to host injury and mortality and, ultimately, assist future vaccine design.  Streptococcus parauberis poses a threat to the dairy and aquaculture industries. With growing human dependence on dairy and aquaculture industries, understanding the pathogenesis of S. parauberis is important to preventing worldwide economic losses, as well as maintaining animal health.