Identificación de las funciones de los manuales escolares

The interaction between the microbial flora of the environment and the avian egg has been extensively researched in the domestic chicken (Bruce and Drysdale, 1994; Baggot and Graeme-Cook, 2002). Microbes have been shown to significantly impact chicken hatching success (Bruce and Drysdale, 1991, 1994). Microbes have the ability to decrease hatching in poultry, though mechanisms such as embryo mortality and alteration of the egg contents (Board, 1968); and increase to hatching through competitive exclusion of egg pathogens (Ribble and Shinefield, 1967).

In wild birds the interaction between microbes, the environment and the egg has been considerably under researched (Baggot and Graeme-Cook, 2002; Cook et al., 2003). Thus, most of the information provided in regards to the causes and impacts of microbial infection on the egg and embryo is based on research on the domestic chicken. However, microbes have been highlighted in many studies as a potentially significant factor in hatching success and failure of wild birds (Cook et al., 2003; Cook et al., 2005a; Cook et al., 2005b; Boyer, 2010; Peralta-Sánchez, 2010; Potter et al., 2013).

2.3.1 Methods of contamination

There are two major ways bacteria can contaminate eggs: contamination via the reproductive tract of the laying bird, occurring prior to oviposition; or trans-shell contamination after the egg is laid (Bruce and Drysdale, 1994).

In chickens, certain poultry and human diseases such as salmonellosishave been shown

to be transmitted vertically through the ovary prior to oviposition. In order for an egg to become contaminated from the mother, a microorganism not only has to survive and persist in the hen, but penetrate and survive in the tissues of the egg-producing organs

and then penetrate and survive in the egg itself (Barrow, 1994). Salmonella spp.,

Mycoplasma spp., Staphylococcus aureus and Pasteurella spp. have been shown to contaminate eggs via the reproductive tract (Mayes and Takeballi, 1983; Humphrey, 1994). In general, there is little evidence for extensive vertical microbial transmission in the domestic chicken. The microbiology of the chicken’s oviduct has been studied multiple times and in all cases the microbial flora recovered from the oviduct differed significantly from that found in eggs (Harry, 1963; Jacobs et al., 1978; Bruce and Drysdale, 1991). This indicates that in healthy hens the majority of contamination occurs after the egg is laid and it is largely accepted in the literature that contamination via the reproductive tract does not have a significant impact on the bacterial species found in the egg (Bruce and Drysdale, 1994). Of the few studies done on wild birds, conclusions are similar; eggs are laid with little infection and that primary source of contamination is the trans-shell route (Cook et al., 2005b).

2.3.2 Factors affecting microbial contamination

Contamination after the egg is laid can be affected by three main factors: the temperature differential between egg and environment; the presence of moisture on the eggs surface; and the presence and type of contamination on the egg and in the environment, including the parent bird (Bruce and Drysdale, 1994; Baggot and Graeme- Cook, 2002).

The temperature differential between an egg and its environment is created when a warm egg cools down (Bruce and Drysdale, 1994; Berrang et al., 1999). Because of the

shell’s porosity, when a warm egg cools a negative pressure is created. This can result in contaminants being drawn through the pores and into the egg contents (Bruce and Drysdale, 1994; Berrang et al., 1999). Therefore avian eggs are particularly vulnerable at the point of laying, as this is when the egg is very warm from the oviduct and has contact with the outside environment and the environment’s microbial flora for the first time (Berrang et al., 1999). The warming and subsequent cooling of eggs also occurs during incubation, when a solo incubating parent leaves the nest to feed (Colbourne, 2002). Temperature also can impact what microbes can grow and survive on and in avian eggs (Board and Tranter, 1995). Whilst incubation temperatures of most birds are the temperatures at which antimicrobial proteins function optimally, they are also similar to optimum growth temperatures of a variety of microbes (Cowan and Steel, 1993).

Another factor affecting microbial contamination is the presence of water, as water is essential for microbial penetration through the pores of the eggshell (Board et al., 1979; Berrang et al., 1999). The level of microbial contamination in chicken eggs has been found to be positively correlated to the amount of moisture present in the air at the time of laying (Graves and MacLaury, 1962; Bruce and Drysdale, 1994). The combination of high humidity and temperature change significantly increases microbial contamination (Haines and Moran, 1940; Board and Halls, 1973; Bruce and Drysdale, 1994; Peralta-Sánchez et al., 2012).

The environment into which the egg is laid is vitally important, as this dictates what microbes are present to contaminate the egg (Bruce and Drysdale, 1994). The environment includes the nest as well as the parental bird’s microbes, and some habitats have more diverse or more numerous microbes than others. High levels of contamination in an environment are usually correlated with high levels of contamination on the egg (Harry, 1963; Baxter-Jones, 1991; La´Baque et al., 2003). For example, soiled nests have high levels of microbial contamination (Graves and MacLaury, 1962; Drysdale, 1985). Dirty eggs, covered in mud and faeces, also have higher microbial contamination than clean eggs (Harry, 1963; Baxter-Jones, 1991; La´Baque et al., 2003). Faecal material is a significant source of microbes and faecal contamination of avian eggshells increases penetration through the shell and shell

membrane (Jahantigh, 2010). In chickens, faecal contamination of eggs is considered to be one of the most important sources of yolk sac infection (Rajesh et al., 2001). The nesting environment can also affect the bacterial load on wild birds. Peralta-Sánchez et al. (2012) found that the eggshell bacterial load of 24 species of birds differenced significantly, both within and between species, over a two year period. They suggest that as well as temperature and humidity, life history traits such as nest type and nest lining affected the bacterial density on avian eggshells.

The presence of external contaminants on the eggshell does not necessarily mean reduced hatchability of the egg. A multitude of factors determine if a microbial species will actually penetrate the shell, grow in the egg contents, and cause problems for the developing embryo (Bruce and Drysdale, 1994). The type of contaminating flora is a major factor in determining if the species will affect the embryo. Only some species have the ability to penetrate the defences of the egg and survive in the adverse conditions of the egg contents (Bruce and Drysdale, 1994). For example; it has been shown that pseudomonads and fungi can digest the cuticle layer, destroy the water resistant properties of the egg and increase the number of unplugged pores available for bacterial penetration (Board and Halls, 1973; Baggot and Graeme-Cook, 2002; Cook et al., 2003). Of the microbes that can overcome the egg’s defences and penetrate into the egg contents, only certain groups appear important in regards to infection and embryo mortality (Bruce and Drysdale, 1991, 1994; Cook et al., 2005b). Some species such as Staphylococcus aureus and some Streptococcus are more effective in reducing hatching

in chicken embryos than for example Enterobacter aerogenes or Micrococcus spp.

(Bruce and Drysdale, 1991, 1994).