Nonfermenting and Other Gram Negative Bacilli

This group comprises Gram negative bacilli that do not ferment glucose, or ferment it only oxidatively rather than through the classic fermentative pathway. They are typically oxidase positive, and are characteristically found in environmental reservoirs, soil, water, moist hospital surfaces, rather than as normal human gastrointestinal flora.

The group is unified by a recurring clinical theme rather than a single taxonomic family: environmental persistence, a propensity for healthcare-associated and biofilm-related infection, and, critically, a pattern of intrinsic, inherent rather than acquired, resistance to many standard antibiotic classes. This makes empirical treatment of suspected infection with these organisms markedly more constrained than for most Enterobacterales.

Morphology. Gram negative bacilli, motile via a polar flagellum, often producing a characteristic blue-green pigment, pyocyanin, and a distinctive sweet, grape-like or corn-tortilla-like odour on culture.

Key laboratory features. Oxidase positive, obligate aerobe, nonfermenting. Grows readily on standard media and is notably tolerant of minimal nutrient conditions, contributing to its environmental persistence.

It is capable of surviving and even multiplying in distilled water, disinfectant solutions, and other nutrient-poor moist environments. This is a recurring source of healthcare-associated outbreaks traced to contaminated equipment or solutions.

Reservoir. A ubiquitous environmental organism, found in soil, water, and moist surfaces generally. It is a minor component of normal human flora in a minority of healthy individuals, moist skin folds, gastrointestinal tract in a small proportion, with colonisation prevalence markedly higher in hospitalised patients and in specific chronic disease populations.

Transmission. Environmental exposure, contaminated water sources including hot tubs and inadequately maintained hospital equipment or solutions, and healthcare-associated spread via contaminated hands, equipment, or surfaces.

Virulence factors. Pseudomonas carries an extensive virulence repertoire reflecting its role as a versatile opportunist:

• pili and adhesins
• exotoxin A, which inhibits protein synthesis via the same elongation factor 2 ADP-ribosylation mechanism as diphtheria toxin, though a structurally distinct toxin
• a type III secretion system injecting multiple effector proteins
• elastase and other tissue-degrading proteases
• prominent biofilm formation, particularly significant in chronic respiratory colonisation

Disease patterns. Pseudomonas is the classic opportunistic pathogen of impaired host defence and disrupted anatomical barriers, causing disease predominantly in specific vulnerable populations and settings rather than in healthy individuals.

In cystic fibrosis, chronic pulmonary colonisation and infection develops, with the organism adopting a mucoid, biofilm-forming phenotype in the CF airway that becomes essentially impossible to eradicate once established, and is strongly associated with progressive lung function decline. This is the single most characteristic organism-disease association for Pseudomonas.

In burns, Pseudomonas is a leading cause of burn wound infection and burn-related sepsis, given the combination of extensive barrier disruption and the organism's environmental ubiquity.

In neutropenia, chemotherapy or haematological malignancy, Pseudomonas is a feared cause of bacteraemia and sepsis, given the organism's virulence and the loss of normal neutrophil-mediated clearance.

Ventilator-associated pneumonia and other healthcare-associated infections in critically ill patients are a further major disease pattern.

Malignant otitis externa is an invasive, potentially skull-base-involving infection of the external ear, classically occurring in elderly diabetic patients.

Hot tub folliculitis is a self-limited skin infection from exposure to inadequately chlorinated warm water.

Ecthyma gangrenosum is a specific cutaneous lesion, a necrotic ulcer with a surrounding erythematous halo, seen in Pseudomonas bacteraemia, particularly in neutropenic patients, reflecting the organism's vascular-invasive tendency.

Diagnosis. Culture from the relevant specimen. The characteristic pigment production and odour are useful preliminary bedside and laboratory clues. Formal identification and, critically, susceptibility testing are required given the organism's variable and often extensive resistance profile.

Resistance patterns. Pseudomonas is intrinsically resistant to a substantial number of standard antibiotics, including most first- and second-generation cephalosporins and many other agents active against typical Enterobacterales. Only a limited number of antipseudomonal agents are reliably active:

• specific antipseudomonal penicillins
• ceftazidime and cefepime among the cephalosporins
• carbapenems excluding ertapenem
• aminoglycosides
• fluoroquinolones
• colistin as an agent of last resort

Further acquired resistance, including carbapenem resistance via multiple mechanisms, efflux pump overexpression, porin loss, and, less commonly than in Klebsiella, carbapenemase production, is an increasing and clinically significant concern.

Morphology. Gram negative coccobacilli, nonmotile, can appear Gram variable on staining, sometimes retaining crystal violet unusually well for a Gram negative organism, a recognised source of identification confusion.

Key laboratory features. Oxidase negative, a notable and useful distinguishing feature from most other organisms in this nonfermenting group, which are typically oxidase positive. Aerobic, nonfermenting.

Reservoir. Environmental, soil, water, and increasingly recognised as a significant healthcare environment coloniser.

It has a particular capacity to persist for extended periods on dry inanimate surfaces, bed rails, equipment, curtains, markedly longer than most other Gram negative bacteria. This is a key factor in its efficiency as a healthcare-associated outbreak organism.

Transmission. Healthcare-associated, via contaminated surfaces, equipment, and hands, given the organism's marked environmental persistence. Historically associated with combat-related wound infections and military treatment facility outbreaks, a notable epidemiological pattern.

Virulence factors. Relatively modest intrinsic virulence compared to Pseudomonas. The organism's major clinical significance lies overwhelmingly in its resistance profile and environmental persistence rather than in a distinctive toxin or invasion mechanism.

Disease patterns. Predominantly healthcare-associated infection in critically ill, ventilated, or otherwise vulnerable patients: ventilator-associated pneumonia, bloodstream infection, often catheter-related, wound infection, and, less commonly, meningitis, particularly post-neurosurgical.

Diagnosis. Culture. The coccobacillary morphology and oxidase-negative result help distinguish it from Pseudomonas and other nonfermenters within the laboratory workup.

Resistance patterns. Notorious for extensive, frequently multidrug or even pandrug resistance, including widespread carbapenem resistance. This makes Acinetobacter baumannii one of the most challenging organisms in contemporary healthcare-associated infection management, frequently requiring combination therapy or reliance on last-resort agents such as colistin.

Morphology. Gram negative bacilli, motile.

Key laboratory features. Oxidase negative, aerobic, nonfermenting.

Reservoir. Environmental, water, soil, moist surfaces, and, as with Pseudomonas, capable of colonising hospital equipment and solutions.

Transmission. Healthcare-associated, environmental exposure, particularly relevant in patients with prolonged hospitalisation, mechanical ventilation, or indwelling devices.

Virulence factors. Relatively low intrinsic virulence, behaving predominantly as an opportunist in already compromised patients rather than a primary invasive pathogen.

Disease patterns. Healthcare-associated pneumonia, particularly ventilator-associated, bacteraemia, often catheter-related, and, similar to Pseudomonas, chronic respiratory colonisation in cystic fibrosis patients, though generally considered less aggressive in this context than Pseudomonas.

Diagnosis. Culture, requiring specific laboratory recognition given its distinctive and clinically critical resistance profile, which markedly affects empirical treatment choice if the organism is not correctly identified.

Resistance patterns. Intrinsically resistant to carbapenems, a specific point that distinguishes it sharply from the general pattern of most other Gram negative organisms, where carbapenems are typically a broad-spectrum, reliable option. Inappropriate empirical carbapenem use in a patient actually infected with Stenotrophomonas is a recognised cause of treatment failure. Co-trimoxazole, trimethoprim-sulfamethoxazole, is generally the treatment of choice.

Morphology. Gram negative bacilli.

Key laboratory features. Oxidase variable depending on species, aerobic, nonfermenting.

Reservoir. Environmental, notably including moist soil and water in specific tropical and subtropical regions for Burkholderia pseudomallei, the cause of melioidosis, endemic particularly in Southeast Asia and northern Australia. Burkholderia cepacia complex is more broadly environmental and healthcare-associated.

Transmission. Burkholderia pseudomallei is transmitted by direct contact with contaminated soil or water, particularly relevant to agricultural exposure and injuries in endemic areas, by inhalation, and rarely by ingestion. Burkholderia cepacia complex is transmitted through healthcare-associated exposure, contaminated solutions and equipment, and person-to-person spread specifically relevant among cystic fibrosis patients, where cross-infection control, physical segregation of colonised patients, is a specific, actively managed clinical practice.

Disease patterns. Burkholderia pseudomallei causes melioidosis, a disease of protean clinical presentation ranging from localised skin and soft tissue infection to severe pneumonia and disseminated, often fulminant sepsis, particularly in patients with diabetes or other chronic disease. It is notorious for a capacity for prolonged latency with reactivation, sometimes remote in time and geography from the original exposure, a recognised phenomenon in military veterans previously deployed to endemic regions.

Burkholderia cepacia complex causes chronic respiratory colonisation and infection in cystic fibrosis patients, associated in some cases with a fulminant, rapidly progressive deterioration termed cepacia syndrome, and is a specific reason for infection control segregation policies among CF patients in clinical care settings.

Diagnosis. Culture, though Burkholderia pseudomallei requires specific laboratory awareness and biosafety precautions given its potential severity and, in some jurisdictions, its status as a select agent of biosecurity concern.

Resistance patterns. Both groups show substantial intrinsic resistance to multiple antibiotic classes, requiring specific, often prolonged combination antibiotic regimens.

Melioidosis characteristically requires an extended eradication phase of treatment, often several months, following initial intensive intravenous therapy. This is given the organism's capacity for latent, relapsing disease if treatment is inadequate.

Morphology. Gram negative bacilli, poorly visualised on standard Gram stain, staining weakly, requiring silver stain or specific culture and antigen methods for reliable detection.

Key laboratory features. Fastidious, requires specialised media, buffered charcoal yeast extract agar, supplemented with cysteine and iron, and does not grow on standard blood or MacConkey agar, a key point explaining why routine sputum culture will not detect this organism unless specifically requested.

Reservoir. Environmental water systems, particularly warm, stagnant, or poorly maintained water sources: cooling towers, air conditioning systems, hot water tanks, decorative fountains, and hospital plumbing systems. The organism is intracellular within environmental amoebae in its natural habitat, a niche that also underlies its capacity for intracellular survival within human alveolar macrophages during infection.

Transmission. Inhalation of contaminated aerosolised water droplets. Notably, no person-to-person transmission occurs, an important epidemiological point distinguishing it from most other causes of severe pneumonia, and relevant to infection control practice: standard precautions are sufficient, isolation is not required.

Virulence factors. Intracellular survival within alveolar macrophages, achieved by preventing phagosome-lysosome fusion, and a type IV secretion system injecting effector proteins that manipulate host cell trafficking.

Disease patterns. Legionnaires' disease is a severe, often multilobar community-acquired pneumonia, classically associated with additional extrapulmonary features atypical for most bacterial pneumonias: gastrointestinal symptoms, diarrhoea, abdominal pain, confusion or other neurological symptoms, relative bradycardia, and characteristic laboratory findings including hyponatraemia and deranged liver function. This cluster of features should raise specific clinical suspicion for this organism over more typical bacterial pneumonia causes.

Pontiac fever is a separate, self-limited, milder flu-like illness without pneumonia, representing a different clinical manifestation of exposure to the same organism, without invasive lung infection.

Diagnosis. Urinary antigen testing is the standard rapid diagnostic method in clinical practice. It detects the most common serogroup, serogroup 1, responsible for the substantial majority of clinical disease, though this means the test can miss infection caused by other serogroups. Culture on buffered charcoal yeast extract agar remains the gold standard but is slow and not routinely performed unless specifically requested. PCR is increasingly available and does not share the serogroup-1 limitation of urinary antigen testing.

Resistance patterns. As an obligate intracellular pathogen during infection, beta-lactam antibiotics, despite often appearing active in vitro, are clinically ineffective because they do not achieve adequate intracellular concentration to reach the organism within host cells. Effective therapy requires agents with good intracellular penetration, specifically macrolides or fluoroquinolones, a critical point directly linking the organism's intracellular lifestyle to a specific, non-obvious treatment implication.

Prevention. Water system maintenance, temperature control, regular disinfection, minimising water stagnation, in buildings, particularly healthcare facilities and hotels, is the principal public health prevention strategy, reflecting the organism's purely environmental transmission route.