Bacteriology 3: Acid-fast bacteria & Spirochetes

Acid-fast bacteria

Bacteria with mycolic acid-rich cell walls that resist conventional staining and grow slowly

What type of bacteria are categorized as acid-fast bacteria?

Mycobacterium

Two diseases caused by Mycobacterium

1.) Tuberculosis

2.) Paratuberculosis (Johne's disease)

Tuberculosis

Chronic, slowly progressive infection characterized by granuloma formation, long latency, and possible respiratory disease

Bacterium responsible for bovine tuberculosis

Mycobacterium bovis

Four major clinical characteristics of tuberculosis

1.) Chronic pneumonia

2.) Persistent fever

3.) Weakness

4.) Emaciation

Pathognomonic lesion of tuberculosis

Tubercle (large granuloma)

Tubercle

Large granuloma formed by the host's immune response to persistent infection

Tuberculosis often remains _______ for long periods of time because...

latent; cell-mediated immunity controls bacterial replication without necessarily eliminating the infection

Five steps in tuberculosis pathogenesis

1.) Inhalation and phagocytosis by alveolar macrophages

2.) Bacteria survive and multiply within macrophage phagosomes

3.) Cell-mediated immune activation stimulates persistent inflammation and formation of tubercles

4.) Large granulomas develop caseation necrosis

5.) Organ damage and cachexia

Initial site of infection for inhaled M. bovis

Alveolar macrophages

Mycobacteria survive and replicate within _______ of macrophages during infection of M. bovis

phagosomes

Mycolic acid stimulates the formation of _______ to help contain infection during infection of M. bovis

granulomas (tubercles)

Caseation necrosis

Cheese-like tissue death within granulomas

Large tubercles can develop caseation necrosis because...

insufficient blood and oxygen supply causes tissue death

Cachexia

Severe weight loss associated with chronic inflammation and altered metabolism

Chronic Mycobacterium infection causes _______ through prolonged release of _______ and _______

cachexia; TNF-α; IL-1

Most tissue damage in tuberculosis results from the host's _______ response rather than direct bacterial toxins

immune

Three major virulence factors of Mycobacterium

1.) Mycolic acid

2.) Cord factor

3.) Sulfolipid

Mycolic acid

Lipid-rich cell wall component that provides resistance to host defenses and stimulates granuloma formation!

The Mycobacterium cell wall contains approximately _______% lipid, contributing to resistance

50

Cord factor

Mycolic acid-containing molecule that promotes bacterial adherence and serpentine cord formation

Cord factor promotes bacterial _______ and characteristic _______ growth

adherence; serpentine cord

Sulfolipid

Virulence factor that inhibits phagosome-lysosome fusion

Sulfolipid allows Mycobacterium to survive inside macrophages by preventing...

fusion of phagosomes with lysosomes

*Facultative intracellular parasitism!

Facultative intracellular parasitism

Ability to survive and replicate within host cells, particularly macrophages; characteristic of Mycoplasma!

Two major diagnostic methods for tuberculosis

1.) Acid-fast staining

3.) Tuberculin skin test

Acid-fast stain used to identify Mycobacterium

Ziehl-Neelsen stain; Mycobacteria appear red with Ziehl-Neelsen staining

Culture isolation of Mycobacterium is difficult because...

the bacteria grow very slowly!

Most practical field screening test for bovine tuberculosis

Tuberculin skin test

Tuberculin skin test

field screening test that detects delayed-type IV hypersensitivity to mycobacterial antigens

A positive tuberculin skin test produces what result?

skin thickening and induration

The tuberculin skin test detects a type _______ hypersensitivity reaction

IV (delayed-type)

The tuberculin skin test is preferred for large-scale screening because...

it is practical for testing large numbers of animals without requiring slow bacterial culture

Three bacterial diseases controlled by testing and slaughtering in the US

1.) Bovine tuberculosis

2.) Bovine brucellosis

3.) Equine glanders

Testing and slaughtering can be effective even when animals have no clinical signs because...

infected animals may remain latent carriers and serve as sources of infection

Paratuberculosis (Johne's disease)

Chronic intestinal infection of ruminants characterized by proliferative enteritis, diarrhea, and emaciation

Bacterium responsible for paratuberculosis (Johne's disease)

Mycobacterium avium subsp. paratuberculosis

Three major clinical characteristics of Johne's disease

1.) Chronic diarrhea

2.) Progressive emaciation

3.) Weakness

Johne's disease has a long latent period of approximately ___-___ years

1-7 years

Pathognomonic lesion of Johne's disease

Proliferative enteritis

Proliferative enteritis

Thickening and wrinkling of the intestinal mucosa and submucosa due to chronic inflammation

Animals with Johne's disease develop chronic diarrhea because...

intestinal thickening and inflammation impair normal intestinal function

Two major differences between tuberculosis and paratuberculosis

1.) Tuberculosis primarily affects the lungs, while paratuberculosis affects the intestines

2.) Tuberculosis produces tubercles, while paratuberculosis produces proliferative enteritis

Spirochetes

Thin, helical, highly motile bacteria that move through tissues using corkscrew-like motion

How do spirochetes move through tissues?

they use axial filaments (modified flagella) and produce a corkscrew movement

Axial filaments

Modified flagella that produce corkscrew-like motility in spirochetes

Three major spirochetes:

1.) Borrelia burgdorferi

2.) Leptospira interrogans

3.) Brachyspira hyodysenteriae

Spirochetes are highly ________ due to their axial filaments

motile

The slender shape and corkscrew motility of spirochetes allow them to...

penetrate tissues and migrate to relatively immunologically protected sites

Lyme disease

Zoonotic tick-borne infection that can cause fever, arthritis, and neurologic disease

Bacterium responsible for Lyme disease

Borrelia burgdorferi

Three major clinical signs of Lyme disease

1.) Fever

2.) Arthritis

3.) Neurologic disorders

Characteristic skin lesion associated with early Lyme disease in humans

Bull's-eye rash (erythema migrans)

Lyme disease is often recognized in animals during the _______ stage because...

chronic; the characteristic rash is difficult to identify beneath their hair coats

Three major mechanisms contributing to chronic Borrelia infection

1.) Corkscrew motility and migration to protected sites

2.) Antigenic variation

3.) Persistent inflammation induced by surface lipoproteins

Corkscrew motility allows Borrelia to...

move rapidly through tissues and reach relatively immunologically protected sites

Antigenic variation

Ability to change expressed surface antigens to evade recognition by the host immune system

Borrelia avoids antibody-mediated immune clearance by changing its surface _______

lipoproteins

What does antigenic variation allow bacteria to do?

cause chronic infection despite antibody production by expressing new surface lipoproteins that existing antibodies may not recognize

Borrelia can persist in relatively protected sites such as...

joints, brain, heart, and kidneys

Borrelia lacks _______ and does not produce _______

LPS; exotoxins

Most reliable serologic test emphasized for Lyme disease

Western blot

Western blot detects _______ against specific Borrelia proteins

antibodies

Culture isolation of Borrelia is difficult because...

specialized media and growth conditions are required

Gram staining is not useful for identifying spirochetes because...

they are extremely thin and difficult to distinguish from surrounding tissues

A tick-borne disease panel may be necessary in suspected Lyme disease because...

ticks can transmit multiple pathogens simultaneously

Primary reservoir of Borrelia burgdorferi

Wild rodents

Vector responsible for Lyme disease transmission

Ixodes ticks (deer ticks)

Three steps in Borrelia transmission

1.) Ticks acquire bacteria while feeding on infected rodents

2.) Infected ticks feed on susceptible animals

3.) Bacteria enter through the tick bite and spread through the body

Leptospirosis

Zoonotic bacterial infection characterized by systemic illness, kidney involvement, and possible reproductive disease

Bacterium responsible for leptospirosis

Leptospira interrogans

Four major clinical signs of leptospirosis

1.) Fever

2.) Anemia and icterus

3.) Nephritis

4.) Abortion

Major virulence factor of Leptospira

β-hemolysin

β-hemolysin contributes to leptospirosis by...

damaging red blood cells and contributing to anemia

Three phases of Leptospira infection

1.) Leptospiremia

2.) Antibody formation

3.) Leptospiruria

Leptospiremia

Initial phase of infection in which Leptospira circulates in the bloodstream and spreads to internal organs

Leptospira initially enters through _______ or _______

damaged skin; mucous membranes

During leptospiremia, bacteria spread through the blood to organs including the _______ and _______

liver; spleen

Antibody formation phase of leptospirosis

Host antibodies develop and promote phagocytic clearance of bacteria from most tissues

Leptospiruria

Phase of infection in which Leptospira persists in the kidneys and is shed in urine

Leptospira persists in the _______ _______ despite antibody formation because...

renal tubules; the tubular lumen provides a relatively protected location with limited immune-cell access

Leptospiruria contributes to continued transmission because...

bacteria are shed in urine and contaminate the environment

Two major reservoir categories for Leptospira

1.) Wild rodents

2.) Infected domestic and other wild animals

Two major routes of Leptospira exposure

1.) Direct contact with infected urine

2.) Contact with urine-contaminated soil or water

Leptospira enters susceptible animals through _______ ______ or _______ _________

mucous membranes; damaged skin

Swine dysentery

Intestinal disease of pigs characterized by mucohemorrhagic diarrhea

Bacterium responsible for swine dysentery

Brachyspira hyodysenteriae

Characteristic clinical sign of swine dysentery

Mucohemorrhagic diarrhea

Primary site of Brachyspira infection

Crypts of the large intestine

Two major mechanisms contributing to mucohemorrhagic diarrhea

1.) β-hemolysin-mediated tissue damage and red blood cell lysis

2.) Increased mucus secretion from goblet cells

Major virulence factor of Brachyspira

β-hemolysin

β-hemolysin of Brachyspira

causes bloody diarrhea because it damages intestinal tissue and lyses red blood cells

Brachyspira causes increased mucus production because...

infection stimulates goblet cells lining the intestinal crypts

Brachyspira can persist in intestinal _______ because...

crypts; they provide a relatively protected environment with reduced exposure to immune defenses

Two major reservoirs of Brachyspira

1.) Wild rodents

2.) Carrier pigs

Primary route of Brachyspira transmission

Fecal-oral

Brachyspira is transmitted when susceptible pigs ingest...

feed or other material contaminated with infected feces

Wild rodents contribute to swine dysentery transmission by...

shedding bacteria in feces that contaminate pig feed