Pseudomonas, Legionella, Moraxella and Other Gram Negative Bacilli

Question 1

Psedomonas genus

Answer 1

• Gram -
• bacilli
• obligate aerobe
• blue/green
• P. aeruginosa

Question 2

P. aeruginosa

Answer 2

• Among the most deadly of bacterial infections
• Opportunistic infection at variable sites
• Confined to the immunocompromised and debilitated
• High level of resistance to antimicrobials
• Aerobic, oxidase and catalase positive
• Pigments (yellow, blue, rust, fluorescent) – Pyocyanin (blue) unique to P. aeruginosa
• Exopolysaccharide (alginate) polymer can form glycocalyx

Question 3

P. aeruginosa Exotoxins

Answer 3

•Exotoxin A or ExoA (can be induced by low iron levels)

-ADP-ribosylates Elongation Factor-2 (same mechanism as in DT, but much different in manefestation)

•Other enzymes (toxins)

-Phospholipase, collegenase, elastase (a protease)

Question 4

P. aeruginosa Epidemiology

Answer 4

• Extremely hardy -free-living in water, soil, plants
• Survives at 42 oC (High GC content)
• Throat, stool microbiota – up to 10%
• Colonizes patients with cystic fibrosis (CF)
• Contaminates medical environment

– Respirators

– Contact lens solutions

– Medications

– Even some disinfectants

Question 5

P. aeruginosa Pathogenesis

Answer 5

• Access via contamination of vulnerable site, solution
• Immune compromise + antimicrobial resistance
• ExoA – no systemic action like DT
• Quorum-sensing

– Bacteria secrete signal chemicals

– ExoA produced when critical mass reached

Question 6

P. aeruginosa CF

Answer 6

Alginate biofilm/glycocalyx

• Regulatory gene mutations allow overproduction of polymer
• Limits access of complement, phagocytes, antimicrobials
• Quorum-sensing activated by high osmolarity of CF secretions

Question 7

P. aeruginosa Manifestations

Answer 7

• Wound, burn, UTI, eye, hot tubs, swimmers’ ear
• Environmental contamination
• Immunocompromised
• CF
• Vascular –> Necrotic lesions (ecthyma gangrenosum)

Question 8

P. aeruginosa Diagnosis

Answer 8

• Culture

Question 9

P. aeruginosa Treatment

Answer 9

• “Wild type” multiresistant to most antimicrobial classes
• Outer membrane porins are relatively impermeable
• Anti-pseudomonas drugs are prized (newer aminoglycosides, cephalosporins, etc.)

Question 10

Legionella genus

Answer 10

• Gram -
• bacilli
• aerobic
• oxidase and catalase +
• fastidious
• Require L-cysteine, ferric ions (fastidious)
• pH optimum 6.9

•doesn’t Gram stain well (prefer environment inside human macrophages or amoebas!)

• use Silver stain

•L. pneumophila

-multiple serogroups (16)

– Serogroup 1 (Philadelphia strain) still most common

Question 11

Legionnaires’ Disease Epidemiology

Answer 11

• Legionella widely present in domestic and wild aquatic environments

– Live in association with algae and inside amoebae

– Dormant states occur in environment

• Aerosol production allows transmission to humans by inhalation

– Cooling towers

– Grocery market mist sprayers

• Human-human transmission not demonstrated
• Attack rate <5% of those exposed
• Immunocompromised patients
• Faucets, showers, etc.
• Biofilms, dormant nutrientrestricted organisms
• Institutional disinfection

– Old pipes, scale, etc.

– Hot water, chlorine

– Silver, copper ions

Question 12

Legionnaires’ Disease Pathogenesis

Answer 12

Question 13

Legionnaires’ Disease Manifestations

Answer 13

• Severe toxic pneumonia
• Dry cough, scant sputum
• Confined to lung Pontiac fever
• Self-limiting febrile illness
• Probably endotoxin or immune

Question 14

Legionnaires’ Disease Diagnosis

Answer 14

Question 15

Legionnaires’ Disease Treatment

Answer 15

• Beta-lactamase producer
• Erythromycin – original outbreak
• Also - azithromycin, fluoroquinolones

Question 16

Moraxella

Answer 16

• Moraxella is another genus of Gram-negative with variable shapes that include coccobacilli to diplococcus.
• Some species require enriched media, such as blood or chocolate agar.
• Their morphology, fastidious growth, and positive oxidase reaction can result in confusion with Neisseria in the laboratory.
• A medically important member is Moraxella catarrhalis.

-Moraxella catarrhalis, a Gram-negative diplococcus, is found in the oropharyngeal microbiota, and it is an occasional cause of lower respiratory tract infection, acute or subacute sinusitis, otitis media (typically beta lactamase-producing) and community acquired suppurative pneumonia.

Question 17

Burkholderia

Answer 17

•Burkholderia pseudomallei is a saprophyte in soil, ponds, rice paddies, and vegetables located in Southeast Asia, the Philippines, Indonesia, and other tropical areas.

• Infection is acquired by direct inoculation or by inhalation of aerosols or dust containing the bacteria.
• The disease, melioidosis, is usually an acute pneumonia; however, it is sufficiently variable that subacute, chronic, and even relapsing infections may follow systemic spread.

•B. cepacia complex is a group of opportunistic species that have been found to contaminate reagents, disinfectants, and medical devices in much the same manner as does P. aeruginosa.

Question 18

Acinetobacter

Answer 18

• The genus Acinetobacter comprises Gram-negative coccobacilli that occasionally appear sufficiently round on Gram smears to be confused with Neisseria.
• They are most frequently found as contaminants of almost anything wet, including soaps and some disinfectant solutions.
• Pneumonia is the most common infection, followed by urinary tract and soft tissue infections.
• Nosocomial respiratory infections have been traced to contaminated inhalation therapy equipment, and bacteremia to infected intravenous catheters.

Question 19

Aeromonas and Plesiomonas

Answer 19

• The genera Aeromonas and Plesiomonas have bacteriologic features similar to those of the Enterobacteriaceae, Vibrio, and Pseudomonas.
• Their habitat is basically environmental (water and soil), but they can occasionally be found in the human intestinal tract.
• Aeromonas is an uncommon, but highly virulent cause of wound infections acquired in fresh or saltwater. The onset can be as rapid as 8 hours after the injury, and the cellulitis can progress rapidly to fasciitis, myonecrosis, and bacteremia in less than a day.