Recently, scientists have been examining the possible genetic basis for "P. aeruginosa" resistance to antibiotics such as tobramycin. One locus identified as being an important genetic determinant of the resistance in this species is "ndvB", which encodes periplasmic glucans that may interact with antibiotics and cause them to become sequestered into the periplasm. These results suggest a genetic basis exists behind bacterial antibiotic resistance, rather than the biofilm simply acting as a diffusion barrier to the antibiotic. Depending on the nature of infection, an appropriate specimen is collected and sent to a bacteriology laboratory for identification. As with most bacteriological specimens, a Gram stain is performed, which may show Gram-negative rods and/or white blood cells. "P. aeruginosa" produces colonies with a characteristic "grape-like" or "fresh-tortilla" odor on bacteriological media. In mixed cultures, it can be isolated as clear colonies on MacConkey agar (as it does not ferment lactose) which will test positive for oxidase. Confirmatory tests include production of the blue-green pigment pyocyanin on cetrimide agar and growth at 42 °C. A TSI slant is often used to distinguish nonfermenting "Pseudomonas" species from enteric pathogens in faecal specimens. When "P. aeruginosa" is isolated from a normally sterile site (blood, bone, deep collections), it is generally considered dangerous, and almost always requires treatment. However, "P. aeruginosa" is frequently isolated from nonsterile sites (mouth swabs, sputum, etc.), and, under these circumstances, it may represent colonization and not infection. The isolation of "P. aeruginosa" from nonsterile specimens should, therefore, be interpreted cautiously, and the advice of a microbiologist or infectious diseases physician/pharmacist should be sought prior to starting treatment. Often, no treatment is needed. "P. aeruginosa" is a Gram-negative, aerobic (and at times facultatively anaerobic), rod-shaped bacterium with unipolar motility. It has been identified as an opportunistic pathogen of both humans and plants. "P. aeruginosa" is the type species of the genus "Pseudomonas". Identification of "P. aeruginosa" can be complicated by the fact individual isolates often lack motility.