Spectrum of infectious diseases caused by Gram-negative bacteria

Gram-negative bacteria (GNB), more specifically Enterobacteriaceae and non-fermenters such as Pseudomonas aeruginosa, can cause a wide array of infectious diseases.

They can even cause no disease at all, since many of them can colonise the gastrointestinal (GI) tract, the respiratory tract or the skin, for instance.

Urinary tract infections (UTIs)

• The most frequent infections caused by GNB are UTIs.

• Urinary tract infections usually happen when a GNB colonising the GI tract ascends through the urinary tract (endogenous infection). Nevertheless, GNB can also access the urinary tract through healthcare hands and/or devices (exogenous infections).

• The presence of bacteria in the urinary tract is called bacteriuria. Bacteriuria does not equal a UTI, since many people may have asymptomatic bacteriuria. Most patients with asymptomatic bacteriuria do not benefit from antibiotic therapy.

• (Symptomatic) UTI are classified into low UTI (cystitis, prostatitis) and high UTI (pyelonephritis, renal abscess). Some might have systemic signs and symptoms, such as fever, chills, sepsis and some might have only local signs and symptoms (e.g. cystitis).

Intra-abdominal infections

• Many GNB, mainly Enterobacteriaceae, can colonise the GI tract without causing disease. Indeed, the GI tract is their normal habitat.

• Whenever the GI tract is disrupted due to inflammation, ischemia or perforation of any cause, the bacteria usually found in the GI tract can cause harm (infection). Although these infections are usually polymicrobial, Enterobacteriaceae are among the most relevant pathogens involved.

• These infections can be limited to the GI tract wall (diverticulitis, cholecystitis, appendicitis…) but they can spread to the peritoneum (peritonitis) and form abscesses (pus collections) within the abdominal cavity.

• Overall, intra-abdominal infections, especially in the case of abscesses, have high bacterial inoculum (high amounts of bacteria) and antibiotics frequently do not suffice for their management, requiring surgical or percutaneous source control (i.e. abscess drainage).

Respiratory infections

• Both Enterobacteriaceae and non-fermenters (Pseudomonas aeruginosa, Acinetobacter baumannii, Stenotrophomonas maltophilia…) can colonise the respiratory tract and they can cause respiratory infections. This usually happens in hospitalised patients or individuals who have been exposed to antibiotics. 

• Respiratory tract infections caused by GNB such as Enterobacteriaceae or non-fermenters can range from tracheobronchitis to pneumonia, either ventilator or non-ventilator associated. Pneumonia caused by GNB is associated with significant morbidity and mortality.

• Not infrequently, pneumonia is difficult to diagnose since its presentation (clinical and radiological) is nonspecific and GNB can colonise the respiratory tract without causing disease.

Other infections

• GNB can cause infections in several other organs. Most of these infections occur in patients with significant exposure to healthcare or immunocompromised hosts.

• Among these infections are: intravenous catheter associated infections, surgical site infections, and febrile neutropenia, for instance, in the setting of GNB translocation from the GI tract.

In the steps that follow, we will present three case scenarios that summarise the spectrum of infectious diseases caused by GNB.