For understanding the death-causative disease, bacteria were isolated and characterized from dead larvae. Representative strains (HL01 0406-1 to HL01 0406-6) were selected for molecular identification by partial 16S rRNA gene sequencing and a molecular phylogenetic tree was constructed.

Six strains (HL01 0406-1 to HL01 0406-6) were identified in morphology, physiology and biochemistry. The strains were examined and found quite similar in morphology, being Gram-positive (some strains were between Gram-positive and Gram-negative), rod-shaped (some were short rod-shaped or spherical), single or paired with round ends, no endospores, and (0.2 - 0.4) μm × (0.3 - 1.4) μm in size. Some flagella were revealed on electronic micrographs for negatively stained strains. The mol% G+C of the DNA was 64.5. The sequenced 16S rRNA gene of strain HL0l0406-l (GenBank accession No. DQ377339) is 1381 bp in length. They shared high similarity at 95%*98% to that of Microbacterium. The results showed that the isolates is a new species of Microbacterium; the selected representative strain (HL010406-1) has been re-checked by China Center for Type Culture Collection (CCTCC), and designated it as Microbacterium shrimpcida sp. nov. The isolates were probably the pathogenic bacteria of acute larvae death shown in pathogenicity of representative strain (HL01 0406-1) by artificial infection experiment to healthy shrimp and flounder.