Some symbiotic bacteria such as Beta (S. alvi) and Gamma-1 (G. apicola) phylotype of A. mellifera also have functions in protection against disease (Martinson et al., 2012). Thus, these symbiotic bacteria in this study maybe participate in the role of virus or bacteria defense which lead to the decline after infection. Potentially, the gut symbionts confer defense or affect immune functioning. Alternatively, environmental stress could perturb the gut community and lead to higher susceptibility of the hosts (Engel and Moran, 2013). However, a causal relationship is not yet confirmed.