Abstract <p>Biopolymers are bio-based degradable plastics with almost similar properties to synthetic polymers. The biopolymers produced from natural resources have advantages than the oil based non-renewable polymers, because of their biodegradable effect. So for more than 100 PHAs (polyhydroxyalkonates) have been identified from the environmental microbes. The major types of microbial PHAs are polyhydroxyhexanoate (PHH), polyhydroxyoctanoate (PHO), polyhydroxyvalerate (PHV) and polyhydroxybutyrate (PHB). They are biodegradable polymers, have many industrial and medical applications. The present study deals with the isolation and characterization of biopolymer producing bacteria from an estuarine sediment sample. Totally 15 isolates, having distinct morphology were selected and they were checked for PHB production individually through Sudan black B staining and Nile Blue-A plate assay techniques. Among the tested 15 isolates, only six (PHB1 to PHB6) isolates showed positive result with Nile Blue-A PHB granules. Further the PHB production was checked and was extracted by dispersion method of Sodium Hypochlorite and Chloroform. The amount of PHB produced was quantified and among the tested six PHB positive isolates, only one isolate (PHB2) showed maximum PHB production of 30.10%, therefore it was considered as a potent PHB producer. Through molecular characterization by 16s rRNA sequencing study, this potent strain was identified as <i>Bacillus cereus</i> MN383184. Further the antibacterial activity of the PHB produced by <i>B. cereus</i> was checked against five clinical pathogens at various concentrations (20–80 µL). The result indicated that, at highest concentration (80 µL), maximum antagonistic activity (11.5 to 13.5 mm) was observed against the tested pathogens.</p>