Bacterial community associated with the gastrointestinal (GI) tract of aquaculture animals can play important roles in health, nutrition and disease. Compared with the GI tract of aquatic vertebrates such as fish, crustacean GI tract has unique structures and surfaces in different segments that may contribute to differences in the bacterial communities. This study examined the bacterial composition and distribution in different segments along the GI tract and in digesta of wild-caught adult Penaeus monodon using Automated Ribosomal Intergenic Spacer Analysis (ARISA), real-time quantitative PCR and clone libraries of 16S rRNA genes. Thirty-nine bacterial species in four phyla including Proteobacteria (a, b, e, c), Firmicutes, Bacteroidetesand Actinobacteria were represented in the GI tract of adult P. monodon. Proteobacteria comprised over 80% abundance of the bacterial community in most segments of the GI tract, except the middle intestine that was dominated by Firmicutes (~50% abundance). The results also showed that bacterial communities showed significant differences along the GI tract segments, particularly the hindgut (p < .001) with Vibrio and Ferrimonas as dominant genera. The knowledge about the distribution of bacteria could be useful in understanding interaction of commensal bacteria and pathogens in different segments, and its potential influence on the effectiveness of probiotic bacteria in the GI tract of shrimp. K E Y W O R D Sgut microbiota, host-bacteria interaction, intestinal bacteria, P. monodon

The rare actinomycetes strain 2EPS was isolated from soil and analysis of cultural, morphological characteristics, diaminopimelic acid content of its cell wall, and 16S rRNA gene sequence indicates that 2EPS belongs to genus Actinomadura. In addition, neighbor-joining phylogenetic tree also confirmed the relationships of this strain to other members of Actinomadura. A butanol extract with antibacterial activity was purified by reversed-phase chromatography to obtain three bioactive compounds, designated as compounds 1, 2 and 3. The structures of these compounds were determined using spectroscopic analysis ((1)H-NMR and (13)C-NMR) and mass spectrometric analysis (HR-TOF-MS). Compounds 1-3 were identified and found to be the same as those included in the Japanese patent number JP 09227587 for spirotetronate antibiotics and are BE-45722A (1), BE-45722B (2) and BE-45722C (3), respectively. All compounds were active against Gram-positive bacteria (Staphylococcus aureus ATCC 25923, Bacillus cereus ATCC 14579, and B. subtilis ATCC 6633) with low MIC values between 0.08 and 5.0 µg/ml. Moreover, both 1 and 3 also exhibited strong activity, with similar MIC values, against Clostridium perfringens S107 at 0.63 µg/ml and C. difficile 630 at 0.08 µg/ml. These results suggest the identified spirotetronate compounds may have potential in the treatment of Clostridium infections. Overall, this analysis demonstrates that rare actinomycetes are a promising source for discovery of antimicrobial compounds.

Actinomycetes are well recognized as the richest source of bioactive compounds, including clinically important antibiotics, antitumor agents and cell function modulators, and hence of high pharmacological and commercial interest. 1 Amongst this group, members of the genus Streptomyces are the most prolific producers of secondary metabolites, accounting for up to 80% of the bioactive small molecules discovered from actinomycetes. 2 Meanwhile, it is quite notable that further discovery of unknown metabolites from Streptomyces is predicted by the genome analysis: the number of metabolites actually isolated is far more below the number of secondary metabolite biosynthetic gene clusters identified in the whole genomes of S. avermitilis and S. coelicolor. 3,4 As a part of our chemical investigation on microbial secondary metabolites, we reported plant-growth promoting spiroacetals of polyketide origin, 5 a linear polyketide with a dlactone terminus with cytotoxic activity, 6 a polycyclic tetronate with antiinvasive activity 7 and a biosynthetically unprecedented heterocyclic polyketide with antibacterial and antiinvasive activities 8 from Streptomyces. During the course of our continuing effort to discover structurally unique secondary metabolites from these organisms, a new modified peptide was isolated from the culture broth of a soilderived actinomycete strain Streptomyces sp. SPMA113 collected in Thailand. The strain was cultured in A-11M liquid medium, and the whole culture broth was extracted with 1-butanol. The HPLC/UV analysis of the extract using our in-house metabolite database indicated the presence of an unknown compound showing a UV absorption maximum at 260 nm, along with geldanamycins 9 and elaiophylins. 10