Burkholderia: Burkholderia cepacia, mallei, pseudomallei

Burkholderia

The genus Burkholderia includes four species, which were formerly classified as Pseudomonas species. These include Burkholderia cepacia, Burkholderia pseudomallei, Burkholderia gladioli, and B. mallei. B. cepacia and B. pseudom-allei are human pathogens, whereas B. gladioli and B. mallei are not.

Burkholderia cepacia

B. cepacia, previously known as Pseudomonas cepacia, was firstdescribed by Walter Burkholder in 1949. It is the causative agent for bacterial rots in onions. The bacteria were first reported as the causative agent of endocarditis in 1950s. Subsequently, the bacteria are being increasingly reported to be associated with urinary tract infections, catheter-associated bacteremias, and wound infections in humans.

B. cepacia is a Gram-negative, motile bacillus and stains irreg-ularly with Gram staining. It is an aerobe, grows well on nutri-ent agar at 25–30°C. On nutrient agar, the bacteria produce nondiffusible reddish purple pigmented colonies on prolonged incubation. They grow on blood agar; on delayed incubation of 3–4 days, the colonies of B. cepacia die. The bacteria do not grow on DCA agar.

B. cepacia is oxidase positive. It utilizes glucose, lactose, malt-ose, and mannitol with production of acid only. It is ornithine decarboxylase and lysine decarboxylase positive. But arginine dihydrolase is negative. B. cepacia is a motile bacterium due to the presence of polar tuft of flagella.

B. cepacia is an opportunistic pathogen. It is found in moistenvironment surfaces and is associated with nosocomial or hospital-acquired infections. It causes respiratory infections in patients with cystic fibrosis or chronic granulomatous disease and also urinary tract infections in patients with indwelling catheters. It has also been reported to cause pneumonitis, osteo-myelitis, wound infections, and septicemias in patients with contaminated intravenous catheter and endocarditis in drug addicts.

B. cepacia is susceptible to trimethoprim–sulfamethoxazole,chloramphenicol, and ceftazidime. But it is inherently resistant to aminoglycosides, polymyxin, penicillins, and cephalosporins.

Burkholderia mallei

B. mallei, earlier known as Pseudomonas mallei, was first isolatedby Loeffler and Scotch in 1882 from a horse suffering from glanders. The bacterium is known to cause glanders, which is a serious disease of equine animals, but capable of transmission to other animals (goats, dogs, and cats) and humans.

B. mallei is a slender, Gram-negative, small bacillus measur-ing 2.5 mm in length. It stains irregularly, often giving a beaded appearance. It is non-motile. It is an aerobe and facultative anaerobe, which can grow on ordinary culture media. It pro-duces small and translucent colonies, which become yellowish on prolonged incubation. It is relatively biochemically inert, fermenting only glucose.

In animals (such as horses), B. mallei produces two types of clinical syndrome: glandular and farcy. B. mallei can be trans-mitted occasionally to humans from infected animals suffer-ing from glanders. The infection is transmitted by inoculation of injured skin or the mucous membrane with contaminated discharges. The condition is mostly occupational, found among veterinarians and persons handling horses and other equine animals.

In humans, B. mallei may cause acute or chronic infection with that localized to the skin, subcutaneous tissue, or respi-ratory tract. Human cases of glanders are usually rare. Acute infection is characterized as an acute fulminant febrile disease with mucopurulent nasal discharge and severe prostration. Mortality rate is very high. Chronic infection produces local-ized abscesses in the skin or respiratory tract.

Glanders in animals may be diagnosed by a skin test known as Mallein test. The skin test is similar to the tuberculin test demon-strating a delayed hypersensitivity reaction to protein of B. mallei. The condition can also be diagnosed by intraperitoneal inocula-tion of clinical specimens into male adult guinea pigs. The posi-tive test is demonstrated by appearance of swelling of the testes, inflammation of tunica vaginalis, and ulceration of scrotal skin in 2–3 days of inoculation. This reaction is known as Strauss reaction.

B. mallei infection is being documented frequently in partsof Asia, Africa, Middle East, Central America, and South America. B. mallei is usually sensitive to ciprofloxacin, strepto-mycin, novobiocin, gentamicin, imepenem, and sulfonamides. Treatment with these antibiotics is usually given for 1–2 months along with surgical drainage of pus from the affected site. B. mallei is usually resistant to chloramphenicol.

Burkholderia pseudomallei

B. pseudomallei is also known as Whitmore’s bacillus, Malleomyces pseudomallei, and Loefflerella pseudomallei. It is the causative agentof melioidosis (from Greek word, meaning resemblance to dis-temper of asses) in rodents. B. pseudomallei was first described by Whitmore and Krishnaswamy (1902) from a glanders-like disease in humans in Rangol. Subsequently, Whitmore (1913) isolated the bacillus and gave the name Bacillus pseudomallei. Stantum and Felcher (1921, 1925) redesignated the bacteria as Bacillus whit-more responsible for melioidosis in humans. Subsequently, thisbacterium was renamed as Pseudomonas pseudomallei and now it is known as B. pseudomallei. B. pseudomallei is saprophyte found in contaminated water, soil, and plants. B. pseudomallei resembles B. mallei but differs in being motile; also, it liquefies gelatin andforms acid from several sugars. B. pseudomallei causes melioidosis in humans. The condition shows different stages such as acute, local, and chronic infections. The incubation period is variable. It may range from as short as 2 days to as high as too many years.

Acute melioidosis is characterized by development of anodule at the site of inoculation of the bacteria in the skin. The bacteria can subsequently spread, causing secondary lym-phangitis, regional lymphangitis, fever, and myalgia. Acute melioidosis may progress rapidly to acute septicemia with high mortality rate. Acute blood stream infection is most commonly seen in patients with HIV, diabetes, renal failure, etc. The condi-tion results in septic shock.

Pulmonary infection manifests as mild bronchitis to severepneumonia. The condition is associated with high fever, head-ache, chest pain, anorexia, and general myalgia. Nonproductive or productive cough with normal sputum is typical manifesta-tion of this condition.

Chronic suppurative infection is associated with multi-ple caseous or suppurative foci of infection in several organs including joints, skin, lymph nodes, spleen, lungs, liver, and brain. Bacteria remain as intracellular pathogens of the retic-uloendothelial system, which contributes to long latency and reactivation of the infection.

Melioidosis caused by the bacteria is endemic in Southeast Asia. In India, most cases of malleiodiosis are reported from Kerala, Tamil Nadu, Maharashtra, Orissa, West Bengal, and Tripura. Diagnosis of the condition is made by:

·           Demonstration of Gram-negative bacilli in the Gram-stained smears; and also demonstration of typical bipolar safety pin appearance of the bacteria in the methylene blue staining of the clinical specimens, such as exudates.

·           Isolation of B. pseudomallei by culture of urine, sputum, pus, or blood confirms the diagnosis of B. pseudomallei infection.

·           Serology is frequently helpful for diagnosis of melioidosis, especially chronic melioidosis.

·           ELISA (enzyme linked immunosorbent assay) and IHA (indirect hemagglutination) are used to demonstrate specific IgM and IgG antibodies in patients with melioidosis.

·           PCR (polymerase chain reaction) which has also been evaluated to detect B. pseudomallei genome in pus, spu-tum, and other specimens.

B. pseudomallei is susceptible to ceftazidime, which is the drugof choice. Other antibiotics include tetracycline, cotrimoxa-zole, and chloramphenicol. Prolonged therapy with antibiotics is essential to treat the patient.