Bacteriology 3: Opportunists & Gram-negative Aerobes

Obligate pathogen

Bacterium that must infect a host to complete its life cycle and causes disease during infection

Opportunistic pathogen

Bacterium that normally does not cause disease but can cause infection when host defenses are compromised

Two major conditions that allow opportunistic pathogens to cause disease

1.) Immunocompromised host

2.) Unusual route of entry through damaged tissue barriers

Three types of organisms that may become opportunistic pathogens

1.) Saprophytes

2.) Symbionts

3.) Commensals

Saprophytes

Organisms that live on dead or decaying organic matter

Symbionts

Organisms that live in or on a host in a mutually beneficial relationship

Commensals

Organisms that live in or on a host without causing disease

Example of a gram negative opportunistic aerobe bacteria

Pseudomonas aeruginosa

Pseudomonas aeruginosa

Gram-negative opportunistic bacterium that causes suppurative infections when normal tissue defenses are compromised

Suppurative infection

Infection characterized by pus formation

Two major sites commonly affected by Pseudomonas infections

1.) Lungs

2.) Skin and wounds

Four major clinical signs of severe Pseudomonas infection

1.) High fever

2.) Lethargy

3.) Confusion

4.) Septicemia and shock

Characteristic appearance of Pseudomonas infections

Green-colored pus due to bacterial pigment production

Five major virulence mechanisms of Pseudomonas

1.) Pili

2.) Exoenzymes and exotoxins

3.) Type III secretion system

4.) Endotoxin (LPS)

5.) Biofilm formation

Pili

Adhesion structures that allow Pseudomonas to attach to damaged or immunocompromised tissues

Pseudomonas pili preferentially adhere to _______ tissues rather than intact healthy tissues

damaged or immunocompromised

Two important exotoxins/exoenzymes produced by Pseudomonas

1.) Elastase

2.) Exotoxin A

Elastase

Enzyme that degrades elastin in lung tissue, interfering with normal lung expansion and recoil

Pseudomonas elastase interferes with respiration because...

it degrades elastin, reducing the lungs' ability to expand and recoil normally

Exotoxin A Toxin that inhibits protein synthesis in host cells, similar to diphtheria toxin

Pseudomonas exotoxin A has a mechanism of action similar to _______ toxin

diphtheria; Inhibits host-cell protein synthesis

Type III secretion system of Pseudomonas

Injects virulence proteins directly into host cells and stimulates inflammatory responses

Pseudomonas uses its Type III secretion system to stimulate production of _______ and _______ in lung tissue

cytokines; chemokines

Endotoxin (LPS) of Pseudomonas

Gram-negative bacterial component that stimulates inflammation and tissue damage

Biofilm

Protective community of densely packed bacteria embedded in a matrix that reduces susceptibility to antibiotics

Pseudomonas biofilms contribute to chronic infection because...

the protective matrix reduces antibiotic effectiveness and allows bacteria to persist

Pseudomonas infections are particularly associated with compromised _______ immune defenses

innate

Wounds and burns increase susceptibility to Pseudomonas because...

they disrupt normal tissue barriers and expose underlying tissues to bacterial colonization

Prolonged antibiotic administration increases susceptibility to Pseudomonas because...

it disrupts normal microbial competition and allows opportunistic bacteria to proliferate

Three characteristic laboratory findings of Pseudomonas aeruginosa pigments

1.) Gram-negative rods

2.) Oxidase-positive

3.) Production of green pigments

Pseudomonas is oxidase-_______

positive

Two pigments associated with Pseudomonas

1.) Pyocyanin

2.) Fluorescein

*give green colored pus

Pseudomonas can be readily identified in the laboratory because...

it grows easily on routine media and produces characteristic green pigments

Primary environmental reservoirs of Pseudomonas

1.) Soil

2.) Water

Two major routes of Pseudomonas transmission

1.) Contact

2.) Inhalation

Pseudomonas infections are difficult to prevent because...

the bacterium is widespread in soil and water and can survive under minimal nutritional conditions

Vaccine availability for Pseudomonas

No vaccine is available!

Bacteria closely related to Pseudomonas

Burkholderia

Two major pathogenic Burkholderia species

1.) Burkholderia mallei

2.) Burkholderia pseudomallei

Burkholderia mallei causes _______ while Burkholderia pseudomallei causes _______

glanders; melioidosis

Glanders

Severe zoonotic infection primarily affecting horses and other equids, characterized by suppurative granulomas and possible septicemia

Bacterium responsible for equine glanders

Burkholderia mallei

Three major clinical forms of glanders

1.) Pulmonary

2.) Nasal

3.) Cutaneous (farcy)

Farcy Cutaneous form of glanders

characterized by suppurative granulomatous nodules that may rupture and release purulent exudate

Burkholderia mallei spreads through the _______ to produce lesions in multiple tissues

lymphatics

Glanders was eradicated from the United States primarily through...

testing and slaughtering

Testing and slaughtering is effective for glanders because...

B. mallei is an obligate pathogen with a relatively limited host range, rare spontaneous recovery, and serious zoonotic potential

Melioidosis (pseudoglanders)

Infection caused by Burkholderia pseudomallei that produces lesions resembling those of glanders

Bacterium responsible for melioidosis

Burkholderia pseudomallei

Natural reservoir of Burkholderia pseudomallei

Tropical soil and water

Burkholderia pseudomallei is a free-living _______ rather than an obligate pathogen

saprophyte

Burkholderia mallei vs Burkholderia pseudomallei — major distinction

B. mallei is an obligate pathogen causing glanders, while B. pseudomallei is an environmental saprophyte causing melioidosis

One example of a gram negative obligate aerobes

Brucella

Brucella

Gram-negative obligately aerobic coccobacillus that causes chronic reproductive infections and survives intracellularly

Brucellosis

Chronic zoonotic infection characterized by reproductive disease, including abortion, infertility, and orchitis

Four important Brucella species and their primary hosts

1.) B. abortus — Cattle

2.) B. canis — Dogs

3.) B. suis — Pigs

4.) B. ovis — Sheep

Disease caused by Brucella abortus

Bovine brucellosis (Bang's disease)

Three major clinical manifestations of Brucella abortus infection

1.) Late-term abortion

2.) Placentitis

3.) Orchitis

Characteristic reproductive sign of brucellosis in pregnant cattle

Late-term abortion, generally after the fifth month of gestation

Characteristic lesion of brucella abortus

Miliary granulomas

Miliary granulomas

Small granulomatous inflammatory lesions associated with chronic Brucella infection

Brucella infection causes reduced fertility in males because...

orchitis and granulomatous inflammation increase testicular temperature and impair sperm production

Brucella-associated granulomas generally lack _______ and extensive _______

pus; necrosis

Two major reservoirs of Brucella abortus

1.) Infected cattle

2.) Wild ruminants, including bison and elk

Primary route of Brucella transmission between cattle

Ingestion of contaminated feed or water

Brucella can enter humans through _______ during contact with infected animals skin

abrasions

Abortion contributes to Brucella transmission because...

large numbers of bacteria are released in fetal tissues, placentas, and uterine discharges, contaminating the environment

Six major virulence mechanisms of Brucella

1.) Facultative intracellular survival

2.) Type IV secretion system

3.) Brucellosome formation

4.) Erythritol utilization

5.) Miliary granuloma formation

6.) Endotoxin (LPS)

Facultative intracellular bacteria

Bacteria capable of surviving and replicating within host cells

Three major host cell types infected by Brucella

1.) Monocytes

2.) Macrophages

3.) Placental trophoblasts

Brucella avoids host immune clearance by...

surviving and multiplying within host cells, where it is protected from many extracellular immune defenses

Type IV secretion system of Brucella

Injects virulence proteins into host cells and modifies intracellular membranes to create a protected replication compartment

Brucella uses a Type _______ secretion system to create brucellosomes

IV

Brucellosome

Modified rough endoplasmic reticulum-associated compartment that provides a protected environment for Brucella replication

Brucella modifies the host _______ to create brucellosomes

rough endoplasmic reticulum

Miliary granuloma formation

Host inflammatory response that attempts to contain Brucella infection but contributes to chronic tissue damage

Brucella endotoxin is relatively _______ compared with that of many other Gram-negative bacteria

weak

Six major steps in Brucella-induced abortion

1.) Ingestion of contaminated material

2.) Spread through lymphatics and blood

3.) Infection of placental trophoblasts

4.) Intracellular multiplication

5.) Placentitis and placental dysfunction

6.) Abortion and bacterial shedding

Brucella primarily causes abortion through _______ rather than direct fetal toxin production

placentitis

Brucella causes placentitis because...

bacteria multiply within placental trophoblasts and stimulate inflammation that disrupts placental function

Brucella-infected animals may remain _______ carriers and continue shedding bacteria in _______ and _______

lifelong; uterine secretions; milk

Four major laboratory characteristics of Brucella

1.) Gram-negative

2.) Small coccobacilli

3.) Obligate aerobes

4.) Some require serum-enriched media and increased CO₂

Two practical field screening tests for brucellosis

1.) Milk ring test

2.) Rose Bengal card test

Milk ring test

Antibody agglutination test that detects anti-Brucella antibodies in milk

A positive milk ring test produces a _______ ring at the top of the milk because...

colored; antibodies agglutinate stained bacterial antigens that rise with the cream layer

Rose Bengal card test

Serologic screening test that detects anti-Brucella antibodies through visible agglutination

A positive Rose Bengal card test produces _______ of stained bacterial antigens

agglutination

Milk ring test vs Rose Bengal card test — sample used

Milk ring test: milk

Rose Bengal card test: serum

Two additional methods used to identify Brucella

1.) PCR

2.) Phage typing

Phage typing identifies Brucella species or strains based on...

susceptibility to specific bacteriophages

Brucella infections are difficult to treat with antibiotics because...

bacteria survive intracellularly, requiring antibiotics capable of penetrating host cells