They were grown on AIA and used in the bioassay with the test microorganisms listed (Table 1). One hundred
and fifteen strains were able to grow well and showed consistent inhibitory activity against the B. marisflavi strain. In addition, nearly one-third of them were active against the previously isolated bee indigenous B. pumilus and B. cereus strains. The growth of Bacillus subtilis was inhibited by about the same number of the actinomycete strains. Two human pathogens, vancomycin-resistant Enterococcus faecium and methicillin-resistant Staphylococcus aureus, were also used as the test organisms in screening for the antibacterial activities produced by the actinomycete isolates. More than one quarter of the isolates clearly produced antibacterial
substances that inhibited the growth of the two human anti-PD-1 antibody pathogens under the assay conditions. We also attempted to test the inhibition of Paenibacillus larvae, the causative agent of hive disease American foulbrood. Because of its much slower growth rate MAPK Inhibitor Library on the MH agar in the bioassay, the agar diffusion assay method failed to show a clear antagonism between an actinomycete isolate and the Paenibacillus strain. Nonetheless, the important confirmation in the second-round screening was that none of the revived strains produced anti-E. coli activities. The frequent occurrence of anti-Bacillus bioactivity from insect IKBKE gut actinomycetes is especially notable in comparison to the bioactivities produced by actinomycetes from soil samples collected in this geographic region. When the same procedures were used to isolate soil actinomycetes, we found more colonies producing anti-Gram-negative and broad-spectrum antibiotics (together 60–70%) than anti-Bacillus-specific producers (30–40%) (S. Chen, unpublished data). Therefore, there appeared to be an unusual enrichment of actinomycetes producing anti-Gram-positive bacteria activities in honeybee guts. This observation is reminiscent of early reports that the bioactivities of Streptomyces isolates from earthworm guts were all
against Gram-positive bacteria (Kristufek et al., 1993). It could also explain a number of reports indicating that the antimicrobial activity of honey products is mainly against Gram-positive bacteria [reviewed in Viuda-Martos et al. (2008)]. One actinomycete isolate named BE74, which consistently produced inhibitory activities against the B. marisflavi strain, was noticed because of its distinctive colonial morphology on the AIA – it appeared waxy, and only started to generate very thin aerial mycelia and poorly sporulate after 5–7 days of incubation. It produced abundant white aerial mycelia and spores when growing on MS agar, but exhibited much less exuberant growth on ISP1 agar. The substrate mycelia on MS agar are brown to yellow. Diffusible brown and light yellow pigments were observed on MS and AIA agars.