34 Tirasak Pasharawipas
STRATEGIES OF PHAGE THERAPY
Logically, it is more promising to use virulent phage to treat bacterial
infection, especially in human and animals (Soothill,1992; Kumari et al., 2009;
Wang et al., 2006), and contamination in food products (Leverentz et al.,
2001; Wall et al., 2010). This is because any progeny of the virulent phages
can be reproduced and attack the other regenerated bacterium again without
the prevention of super-infection by the bacterium. On the other hand, the
temperate phage can naturally cause the target bacterium to tolerate the same
phage infection to produce either lysogen or pseudolysogen.
There are reports that there are more temperate phages than virulent
phages in our body. Oppositely, there are more virulent phages than temperate
phages in natural environment (Chibani-Chennoufi, 2004a). However, we
studied the phage of Vibrio harveyi (VH), which is the pathogenic bacterium
of black tiger shrimp. Although over 20 strains of temperate VH phages have
been isolated, we have never been able to isolate a virulent phage against VH
from any shrimp pond. As mentioned above, it is still possible to use
temperate phage to treat bacterium contamination in shrimp pond if the
sufficient MOI of the temperate phage was applied to kill the bacterium at
once. Insufficient MOI of temperate phages can induce the bacterium to
develop lysogenic bacterium which can tolerate the same phage super-
infection. So, the same temperate phage cannot be used to eliminate the VH
bacterium any longer (Pasharawipas et al., 2011).
Accordingly, using temperate phage as an anti-bacterium tool can be
disadvantageous compared to the use of virulent phage. The application of
temperate phage is not proven to be appropriate for the study or treatment
within the body especially in human since the phage genome might convert the
normal flora to a pathogenic bacterium (Dublanchet and Fruciano, 2008). This
phenomenon is called phage conversion which is a mechanism to convert non–
pathogenic to a pathogenic bacterium by the phage genome as reported in
Vibrio cholerae (Campos et al., 2010; Hansan et al., 2010), Staphylococcus
aureus (Endo et al., 2003) Salmonella enterica (Brown et al., 1999), and
others (Dobrindt and Reidl, 2000). However, many believe that phage
conversion is unlikely to occur since the phage requires a specific bacterium
host to infect. It is not very promising that phage can infect normal flora unless
the phage for some bacterial species is used such as the phage to treat
pathogenic E. coli. It is known that in using phage to treat pathogenic E. coli,
we should be aware. E. coli is a normal flora in our gastrointestinal tract. On
the other hand, using the phage to treat the infectious agents such Vibrio