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closeMusca domestica and Calliphora vicina: different families and different physiology
Posted by antoniofasanella on 15 Jan 2014 at 11:41 GMT
Many thanks to the authors of the article entitled “Failure of Sterne- and Pasteur-like strains of Bacillus anthracis to replicate and survive in the urban bluebottle blow fly Calliphora vicina under laboratory conditions” because they give us the opportunity to clarify some points of the article entitled “Evaluation of the House fly Musca domestica as a Mechanical Vector for an Anthrax . (2010) : PLoS ONE 5 (8)” which is cited several times.
The observations of the authors are correct in the point where they write: However, germination in animal blood starts irreversibly within minutes of inoculation of dormant spores leading to non-heat resistant spores (Carr et al., 2010). Such spores, if taken up by flies, may develop into encapsulated vegetative cells which, as indicated by our results and previous data from the literature, do not replicate within the fly gut” . It is known that in animal blood germination starts within minutes. In the article entitled “Evaluation of the House fly Musca domestica as a Mechanical Vector for an Anthrax . (2010) : PLoS ONE 5 (8)” we didn’t report that the blood was also taken on a small refrigerated support to maintain a temperature below that required for germination.
However it is well known that the two species, i.e. Musca domestica and Calliphora vicina, belong to different families, Muscidae and Calliphoridae, respectively. This means that they differ in physiology reflecting their phylogenetic distance ( Shaw et al., 2001). In both species, the midgut (the ventriculus and the proximal intestine) is the main organ involved into the fly’s digestion process (Greenberg, 1973). There is a substantial difference related to the pH: the adult midgut in Musca domestica ranges from 7.2 to 7.6, while adult midgut of Calliphora vicina is significantly lower, i.e. 2.8. Thus, the former pH appears much more close to that the investigated microorganism needs to replicate (pH 6 to 7.5) and therefore compatible with B. anthracis growth.
More importantly is to underline that it has been long shown that such pH in Calliphora vicina causes a 5-log destruction of Gram+ and Gram- microorganisms in less than 10 minutes (Greenberg, 1968).
Based on these data, it may be hypothesized that nature has given to Calliphora the role to control the increases of the environmental bacterial contamination
References
Greenberg, B. (1968): Micro-potentiometric pH determinations of muscoid digestive tracts. Ann. Entomol. Soc. Amer., 61: 365- 368.
Greenberg, B. (1973): Flies and disease. Vol.11, Biology and disease transmission. Princeton Univ. Press, Princeton, New Jersey.
Shaw J, Salameh A, McGregor A., Bala S, Dover G. 2001. Divergent structure and function of the bicoid gene in Muscoidea fly species. Evolution & Development 3:4, 251–262
Antonio Fasanella1 and Martin Hugh Jones2
1 Istituto Zooprofilattico Sperimentale della Puglia e della Basilicata, Anthrax Reference Institute of Italy, Foggia, Italy
2 Department of Environmental Sciences School of the Coast & Environment, Louisiana State University, Baton Rouge, LA 70803-5703, USA
RE: Musca domestica and Calliphora vicina: different families and different physiology
beyerw replied to antoniofasanella on 15 Jan 2014 at 15:43 GMT
In his highly appreciated comment to the article entitled “Failure of Sterne- and Pasteur-like strains of Bacillus anthracis to replicate and survive in the urban bluebottle blow fly Calliphora vicina under laboratory conditions” Dr. Fasanella stresses physiological differences between the two species Musca domestica and Calliphora vicina thought to be the reason for different results using these flies in experiments investigating the fate of Bacillus anthracis in the digestive tract of the flies.
Regarding the literature cited to explain the physiological differences I here would kindly add the following thoughts:
The review published by Clark (1999), also referring to the paper of Terra and Regel (1995), states that both the families of Calliphoridae and Muscidae belong to the Infraorder of Muscomorpha, which differ from other Infraorders by their midgut pH. Table 1 in Clark (1999) gives the pH of the midgut for Calliphoridae as 2.8-4.1 and the midgut pH for Muscidae as 3.1-3.9, both cited from Greenberg (1968). Terra and Regel (1995) differentiate into an anterior, middle, and posterior midgut with pH-values of 6.1, 3.1, and 6.8, respectively. They state that “the low pH observed in the middle midgut of M. domestica, and of other cyclorrhaphous flies” (which all Muscoidae, inclusive Calliphoridae belong to) “deserves special attention because those insects are the only animals,..., to display such an acidic region in their guts...”
While none of the authors of our publication claim to be experts in fly entomology it becomes clear from this literature that M. domestica and C. vicina should be very similar in their gut conditions. I therefore do not see a good reason at this point that spores of B. anthracis behave differently in the two fly species as a consequence of different pH values in their midguts.
Moreover, it should be noted that all the pH measurements published in the literature are drawn from fly larvae, not from imagos. According to a personal communication with a specialist from the entomological institute of the University of Hohenheim, the gut conditions of the maggots are not identical to those of an adult fly, which makes statements regarding the actual conditions in the gut of an adult fly questionable.
Reference:
Clark, T.M. (1999): Evolution and adaptive significance of larval midgut alkalinization in the insect superorder Mecopterida. J. Chem. Ecology, 25:1945-1969.
Terra, W.R., Regel R. (1995): pH buffering in Musca domestica midguts. Comp. Biochem. Physiol., 112A:559-564.
With regard to the question of whether some germination happened in the blood culture used to feed Musca domestica in the former paper by Fasanella et al. (2010), PLoS ONE 5 (8), I appreciate the explanation added in the comment by Dr. Fasanella. The discussion in our paper simply arised because the ratio between dormant and germinating spores in the blood meal was not provided in the paper. Germination, as determined by the presence of encapsulated cells in the gut of 7 out of 20 flies tested could, therefore, have been a result of germinating spores in the blood meal.
Wolfgang Beyer
University of Hohenheim, Institute of Environmental and Animal Hygiene, Garbenstraße 30, 70599 Stuttgart, Germany