have been employed to differentiate P. penneri from P. vulgaris and P. mirabilis strains. The RAPD technique, fundamentally a DNA fingerprinting method, has exposed a substantial DNA diversity among P. penneri strains, a characteristic that remained unidentified by other methods. Subtypes The lipopolysaccharide (LPS) core region of P. penneri strains contains higher structural variability than that observed in other representatives of Enterobacterales. These differences have been used to cluster P. penneri strains into serogroups based on their agglutinating activity when mixed with antibodies directed against specific species of LPS molecules. Presently, 15 O-serogroups have been proposed for P. penneri based on the chemical structure of the O-specific polysaccharide chain (O-antigen) of the lipopolysaccharide. Certain LPS epitopes have been examined to determine their function in antigenic specificity. The particular groups on the oligosaccharides found to play a dominant role in the specificity of P. penneri LPS are the amide of D-galacturonic acid with L-lysine α-D-GalA-(L-Lys) (and the amide of D-galacturonic acid with L-threonine α-D-GalA- [L-Thr]), respectively. Isolation The occurrence of P. penneri organisms in the normal intestine accounts for their higher frequency in urinary tract infections and for their role as opportunistic invaders after surgery. P. penneri is absent from the intestines of livestock. The optimum growth condition for P. penneri is achieved at 37 °C, which mirrors the intestinal niche colonized by many of these bacteria. Certain strains of P. penneri can differentiate into elongated hyperflagelled cells during development on solid media, resulting in the surface translocation event identified as “swarming”. Swarming motility makes it difficult to isolate single colonies for further study. Incidence and epidemiology The proportion of P. penneri strains isolated in clinical specimens is unknown. Since P. penneri was only recently recognized as a new species, many bacteriologists are either generally unaware of it, or have made limited attempts to discover its clinical significance. Thus, reports on the isolation of P. penneri from infected patients is limited. Although increasing numbers of laboratories are now identifying P.penneri strains, the numbers reported in susceptibility studies are relatively small for a general assessment of the incidence of the species. Likewise, the epidemiology of P.penneri is also unknown for these reasons. Clinical significance Documented human clinical infections caused by P. penneri have been limited to the urinary tract and to wounds of the abdomen, groin, neck, and ankle. This species is isolated from individuals in long-term care facilities and hospitals and