vibrios

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**Introduction**
Many members of Vibrio species ecologically interact with other organisms that may affect humans. One species in particular, //Vibrio vulnificus,// has devastating effects on humans as they can cause gastroenteritis or even death (Oliver et al., 2001). Naturally, it is in our interests to determine the factors that affect abundance and distribution of them. Upon studying potential parameters that may produce significant results to their abundance, we can then use such measures to better regulate public health with regards to these infectious species.

**Taxonomy**
Sixty five different species of the genus Vibrio inhabit ubiquitously in the ocean (Oliver et al., 1982). Their environment ranges from freshwater (ie. 0% salinity) to saline waters, and from shallow waters to deep waters as seen on Figure 1 (Oliver et al., 1982). They belong to the kingdom Bacteria, the Proteobacteria phylum, the Gamma Proteobacteria class, the Vibrionales order, and finally the Vibrionaceae family (Oliver et al., 1982). //Vibrio vulnificus// constitutes one species of the genus Vibrio, so it is therefore logical to briefly discuss basics of the Vibrio species in general.

**Physiology**
Most Vibrio species are gram negative, rod-shaped chemo-organotrophs (Pfeffer et al., 2003). They are usually 1 µm in width and 2~3 µm in length (Chan et al., 1986). Their physiology, however, changes depending on the environment. They are able to change surface to volume size in order to adapt to the environment: when the environment is rich in nutrients, their surface area to volume decreases. On the other hand, they increases the surface area to volume ratio in order to scavange for as much nutrients as possible in nutrient-limiting environments (Pfeffer et al., 2003). In general, they are very versatile that some representatives of the Vibrios can withstand the pressure at deep waters, some can live symbiotically with other organisms like molluscs and fish, whereas other species can withstand little to no salinity; hence, the reason why vibrios are found virtually everywhere in the ocean.

**Environment**
Given the kind of environment required for vibrio species to survive, the real question is what kind of environment they thrive in the best. As expected, multiple factors produce different environments and thus the the factors that affect distribution of vibrio species are multifactorial. Like a symphony orchestra, vibrio species require different instruments (factors) to perform well. Some "instruments" are often overlooked and may not seem to contribute much to the overall abundance and distribution; ultimately however, significant or not, every instrument must play at optimal level for success. On average, vibrios require 2~3% of salinity for optimal growth (Kaspar et al., 1993). As mentioned, some species like //V.cholerae, V. mimicus, and V. navarrensis// have adapted to live in freshwater environment (Chan et al., 1986). Most vibrio species require water temperatures above 15°C (Kaspar et al., 1993). Moreover, other factors like number of total bacteria present in the water column, pH, dissolved oxygen level, turbidity, nutrient levels, and depth do have the potential to produce different abundance levels (Olver et al., 1982).

**Introduction**
//Vibrio vulnificus// are most well known for human diseases like wound infection, gastrointestinal disease, and septicemia. It is the most lethal species of vibrios and causes 50% mortality when infected (Kumamoto et al., 1998). Unlike vibrio species in general, //Vibrio vulnificus// are obligate halophiles and require at least 5% of salinity level for growth (Oliver et al., 1983).

Distribution
As figure 1 suggests, //Vibrio vulnificus// are most commonly found in brackish water, where salinity is very low; thus, they are abundant in estuarine and coastal waters where some fresh waters mix with seawater to produce a low salinity level of approximately 5% or above (Oliver et al., 1983).

Factors Affecting Distribution and Abundance
Due to the rising concern for the //Vibrio vulnificus// infection, many researches have been dedicated towards finding exactly what factor, or factors lead to maximal growth. As aforementioned, however, I believe many factors together yield an environment suitable for growth of //Vibrio vulnificus//. Some factors are nevertheless more important than others. The ocean is stratified according to density gradients, of which is defined according to different combination of temperature and salinity levels. These two factors definitely play significant roles in determining the abundance of //Vibrio vulnificus.

Since Vibrio vulnificus are obligate halophiles, they must have water salinty level of at least 5%. // Kaspar successfully showed Vibrio vulnificus are unable to inhabit freshwater (ie. 0% salinity). //In his 1993 paper, a positive correlation was drawn between salinity and abundance - higher the salinity, higher the abundance. This was true for salinity levels ranging from 0 to 25 ppt (Kaspar et al., 1993). At higher levels, however, the abundance and growth declined to more than half the original biomass. Kaspar successfully showed Vibrio vulnificus are unable to inhabit freshwater (ie. 0% salinity). As shown on figure 3, there is an increase of abundance towards the optimal level of 25 ppt, then a drastic decrease at higher levels. This study is further supported by studies from other researchers. Motes et al., 1996 also concluded that salinity levels of higher than 25 ppt limit growth of Vibrio vulnificus.

Temperature is the ultimate, universal source of life on Earth. Making comparison with the orchestra again, sunlight is the conductor, the maestro of all life forms on Earth. Sunlight translates to the degree of temperature in air, and similarly in the ocean as well. The degree of temperature effectively makes a chain reaction, affecting other parameters like pH, degree of mixing, abundance of other organisms that require sunlight; so incidentally, temperature must, and should be responsible for abundance of Vibrio vulnificus. Clear in figure 2, temperature also draws a positive correlation with abundance of Vibrio vulnificus. Numerous studies support this idea too (Chan et al., 1986; Jones et al., 1998; Kaspar et al., 1993; Kelly 1982; Motes et al., 1998; Oliver et al., 1983; Pfeffer et al., 2003). The positive correlation occurs at temperatures above 15°C. At temperatures below this level, Vibrio vulnificus can no longer be isolated (Pfeffer et al., 2003).

This conclusion definitely questions the capability to isolate and identify the Vibrio vulnificus, and other studies have found ways around this problem. Other research data correlates incidence of human infection and isolation from oysters (Motes et al., 1998; Randa et al., 2004). It is certainly possible that at lower temperatures, these bacteria may transform into non-identifiable state (ie. dormancy); however, despite the limitations in actually counting the biomass of the bacteria, such parameters are useful for the purpose of finding abundance of "useful" Vibrio vulnificus. That is, we are only interested in the abundance of Vibrio vulnificus because they can infect humans, and if they are not isolatable (ie. in non-infectious state), then we can safely assume that they are dead or non-existent.

To no surprise, temperature accounts for 47% of the variability in the abundance of Vibrio vulnificus (Pfeffer et al., 2003). The degree of variability is not actually calculated in other studies; however, let's assume that this calculation is true. The next logical question is what about the other 53%? Pfeffer et al., 2003 went on to study other potential parameters may affect the abundance of Vibrio vulnificus. They concluded that parameters like pH, turbidity, salinity, iron concentration, ammonia nitrogen concentration, phosphorous concentration, dissolved oxygen content, and total number of estuarine bacteria count do not yield significant results. This antithetical conclusion that perhaps attempts to outrule every other study and research further questions our capability in isolating and producing accurate results. Do we have the necessary tools to define the effects of "non significant" parameters on Vibrio vulnificus ? How is it that different studies produce different conclusions?

In my opinion, they do not have the necessary means to observe the effects of the potential factors other than temperature - they have the wrong approach! Working with temperature may be have been an easy parameter to study since it shows clear and significant results. I disagree, especially with Pfeffer et al., 2003, with the approach of finding the multiple factors that affect abundance. I believe all these parameters are linked to one another primarily to temperature. The temperature is like a keystone species in a food web; it connects with every potential factor. For example, dissolved oxygen content is negatively correlated with temperature, and thus technically, DOC levels should result in significant changes in the bacterial abundance. Change in DOC level may not necessarily mean change in temperature, however, it does mean there has been a change in temperature. Change in temperature clearly affects the abundance of Vibrio vulnificus ; so by the inverse proportionality of these two parameters, change in DOC affects bacterial abundance.

Pfeffer failed to provide data that supports significant effects of salinity to bacterial abundance. This is in disagreement with many other studies; for example, the Oliver et al., 1983 paper. Furthermore, Oliver et al. found correlations between the abundance of Vibrio vulnificus and the pH level. This may mean since 1983, the advancements in technology allowed Pfeffer to produce more accurate results; however, Pfeffer failed to provide convincing evidence supported with concrete data that correctly identifies what factors affect abundance of Vibrio vulnificus - that is, what accounts for the other 53% of the variability.

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Conclusion
Some things cannot be helped due to the limitless diversity in environments world-wide. Different studies are done in different sites, and every study may have different numbers and results. Their conclusions, however, should relate to one another - they should agree with each other. With the advances in genetic sequencing to identify different strains of the bacteria Vibrio vulnificus, new studies will allow for advancements in observation and producing accurate results. The use of epifluoromicroscopy was a giant technological leap in microbes in the ocean, but there's always room for more improvement and the fact that there are still disagreeing results in different studies is one of many wake up calls (that our current results may not truly reflect what is out there, and that we are not at full understanding yet). Finally, and hopefully, the continuing advancements in technology for observing different parameters and their effects could lead to identifying all parameters that affect their abundance and distribution. Ultimately, we may be able to use such information to not only better understand //Vibrio vulnificus//, but also to suppress the abundance of them artificially. Then, we no longer need to worry about Vibrio vulnificus in our oysters!

**References**

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 * **Motes, M. L., A. DePaola, D. W. Cook, J. E. Veazey, J. C. Hunsucker, W. E. Garthright, R. J. Blodgett, and S. Chirtel.** 1998. Influence of water temperature and salinity on //Vibrio vulnificus// in Northern Gulf and Atlantic Coast oysters. //Appl. Environ. Microbiol//. **64**:1459-1465.
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