NOVELTY - Method for inhibiting growth of a Gram-negative staining bacterium in a subject or substance in need involves administering or applying an antimicrobial composition to the subject or substance, where the antimicrobial composition contains isolated Paenibacillus polymyxa bacterium which has a 16S ribosomal RNA (16SrRNA) gene containing a DNA comprising a 1457 base pair sequence (SEQ ID NO: 1) fully defined in the specification, and cell culture comprising bacterium.    USE - The method is useful for inhibiting growth of a Gram-negative staining bacterium in a subject, preferably human, pet or agricultural animal or substance chosen from cosmetic product, hygiene product, feed product, food product, preferably dairy or meat product and its packaging material, and reducing mortality or morbidity of an animal. The Gram-negative staining bacterium is chosen from Escherichia coli strain RR1, E.coli strain TB1, E.coli strain O157:H7, Pantoea agglomerans strain BC1, Pseudomonas fluorescens strain R73, Butyrivibrio fibrisolvens strain OR85, Fibrobacter succinogenes, Shigella sp., Helicobacter sp., Campylobacter sp. and P.aeruginosa (all claimed).    DETAILED DESCRIPTION - Method for inhibiting growth of a Gram-negative staining bacterium in a subject or substance in need involves administering or applying an antimicrobial composition to the subject or substance, where the antimicrobial composition comprises (a) isolated Paenibacillus polymyxa bacterium which has a 16S ribosomal RNA (16SrRNA) gene containing a DNA comprising a 1457 base pair sequence (SEQ ID NO: 1) fully defined in the specification, (b) isolated P.polymyxa bacterium deposited at ATCC under the terms of Budapest Treaty and designated Accession Number PTA-10436, (c) cell culture comprising the P.polymyxa bacterium, (d) cell culture supernatant derived from growing P.polymyxa bacterium in a cell culture medium, or (e) antimicrobial agent isolated from the P.polymyxa bacterium, the cell culture or the cell culture supernatant. INDEPENDENT CLAIMS are included for the following: (1) method (M2) for producing an antimicrobial agent involves growing or culturing an isolated P.polymyxa bacterium in a cell culture medium to produce a cell culture supernatant comprising the antimicrobial agent; and (2) method (M3) for reducing mortality or morbidity of an animal involves administering antimicrobial composition to the animal.