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Veterinary Bacteriology and Mycology

PJL:2011

Bacterial Overview: Morphology, Structure, Jargon

General Features

Domain Bacteria Proteobacteria

Spirochaetes

Firmicutes

Actinobacteria

No nuclear membrane

Generally 1 chromosome

Extrachromosomal DNA

Oil- immersion (1000x) Cocci : ~ 0.5 – 1.5

Rods: ~

Spirochaetes : 0.5 x 10-20 m

PJL:2011

http://www.cdc.gov/ncidod/eid/vol4no3/relmang.htm

Firmicutes, Actinobacteria

Bacterial Cell Morphology

PJL:2011

Streptococci

Staphylocci

Bacillus sp.

curved rod Branching filamentous

Spiral/coiled - spirochetes

Overview of Bacterial Envelope Structure

Why is Envelope important ?

For the bug: Facilitates a “controlled” interaction with environment (including host)

For us: Important diagnostic tool : Gram stain (Christian Gram , 1884) This stain allows us to determine:

Size, morphology and Gram reaction facilitate presumptive identification and early treatment decisions

Gram stain variations “Old” /Anaerobic /antibiotics - gram-positives appear gram-negative Gram-variable organisms Number of Genera that require special stains

PJL:2011

Classic Bacterial Envelopes

PJL:2011

Corynebacterium-Nocardia-Mycobacterium Group

Gram +ves with a “Gram-negative like” OM

NO LPS

Mycolic acid

+/- porin-like OMPs

Niederweis et. al. , 2010

PJL:2011

Figure 3: Corynebacterium-Nocardia-Mycobacterium

Group Envelope

Capsule

Inner

Membrane

Peptidoglycan

With arabinose

and galactose

Mycolic acid rich

Outer-membrane

Mycolic acids

Porin-like proteins?

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Pathogen-Host

Interactions

Therapeutic - treatment of disease (antimicrobials,

quarantine, culling and destroying)

A medical strategy “limited” to diagnosis and treatment (therapy) there

is virtually not possibility of large scale control or eradication of infectious disease

Prophylactic - prevention of disease (management, vaccination, antimicrobials, identification and eradication)

Infection is a complex interaction between the host, the pathogen and

the environment.

Dr. Mike Collins referred to this as the Beast: The Bug: The Business

Definitions and Concepts

Pathogen - organism that can cause disease

Commensalism: a state of “infection” that results in either no damage or clinically apparent damage to host.

Obligate pathogen - organism that almost always causes disease when present in host (Bacillus anthracis)

Primary pathogen - organism that generally causes disease

Opportunistic pathogen: often commensals which can cause disease when they gain access to a different tissue type (non-enterotoxin producing E. coli gain access to urinary tract). Often these infections arise when host immune defenses are impaired (immunosuppression, stress) – “Shipping fever” in feedlot cattle.

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Definitions and Concepts

Facultative Intracellular - can replicate within host cells as well as extracellular spaces (classically this group included; Listeria, Salmonella, Mycobacteria, Brucella, etc.) . There is an expanding list of bacteria that fall into this category

Obligate intracellular - must access host intracellular space to replicate and survive (Chlamydiales, Rickettsiales, Coxiella burnetti , Lawsonia sp.)

Virulence - degree of pathogenicity that is typically multifactorial (tissue invasion factors, immune escape, toxins, biofilms).

Infection – presence of potentially pathogenic organisms in a host.

Acute, chronic, subclinical

Carrier - an animal which harbours a disease organism without manifesting clinical signs and appears healthy

Bacterial Interaction with Host

Attachment/Tissue Invasion Factors Allow attachment and subsequently colonization

of the host

pili/fimbriae, capsules, and non-pilus type adhesins (ie. Mycoplasma adhesins)

Hyaluronidase (many bacteria) and collagenase (some strains of Clostridium perfringens) breakdown host intercellular materials and aid in further spread of extracellular pathogens

Acquisition of critical nutrients (Fe) – siderophores (gram-negatives)

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Virulence Factors Cont’d

Immune Escape: Utilize a variety of mechanisms to escape detection by

the host’s immune surveillance system Extracellular/secreted products: antiphagocytotic capsules,

coagulase, superantigens, cytotoxins

Possess specific virulence factors to evade the

phagolysosome and thereby spread from cell to cell.

Surface Antigenic Variation (ie.Mycoplasma spp.) foil antibody

recognition.

Induce apoptosis of host cells involved in immune response

(e.g. Salmonella, Histophilus somni)

Virulence Factors Cont’d

Toxins

Exotoxins

Produced by a variety of gram-positive and gram-negative

bacteria

Kill host phagocytic cells, acquire nutrients

Endotoxins

Lipid A component of LPS

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Biofilms and Quorum Sensing

Biofilms – an immune escape strategy

Sessile, structured community of bacteria cells

Inherently resistant to Abx → persistent infections

A growing number of gram-positive and gram-negative pathogens demonstrate this capability.

Disease manifestations include: tooth decay and periodontal disease, otitis media (humans), medical devices

Quorum Sensing: cell-to-cell communication between bacteria of the

same species (e.g. P. aeruginosa and S. aureus).

Mechanisms of Host Resistance

A. Natural or Innate Immunity

Rapid response that involves the recognition of

unique components of bacterial cells (Pathogen-Associated Molecular Patterns - PAMPs) by a variety of host cell receptors (Pattern Recognition Receptors - PRRs).

Activation and recruitment of neutrophils and macrophages to the site of infection.

Frequently influences acquired immunity

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Host Adhesins/Receptors

Acute Phase Proteins

Complement C3 - binds bacterial CHO’s

C-reactive protein (CRP) and Serum amyloid protein (SAP) –

binds bacterial surfaces and activates complement

Ferritin – binds iron to restrict essential nutrient

Lipopolysaccharides (LPS) binding protein (LBP)

Mannan-binding protein (MBL) – binds terminal mannose on

bacteria and activates complement

Macrophage Scavenger Receptors

Binds peptidoglycan (PGN), LPS and lipoteichoic acid (LTA)

Host Receptors Cont’d

Toll-like Receptors (TLRs)

These are PRRs that bind a variety of PAMPs

Expressed by different cell types

TLR -2 (with the aid of TLR-1 and 6) recognize PGN, LTA, LAM, bacterial lipoproteins, LPS (Leptospira)

TLR - 4 binds LPS (LBP-LPS-CD14 complex)

TLR - 5 receptor for bacterial flagellin

TLR - 6 Mycoplasma lipopeptides

TLR - 9 recognizes DNA with unmethylated CpG-motifs, abundant in bacterial DNA

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Cationic Antimicrobial Peptides

Over 500 hundred small peptides (15-20 aa) identified in

virtually all species

Some inducible, some constitutively expressed

As a group they have very broad spectrum of activity that includes: Gram-positive, gram-negative, fungi, parasites

Found in ears, on eyes, skin and mucosal epithelial surfaces etc.

Granules of polymorphonuclear leukocytes (PMN’s)

Mechanisms of Host Resistance

B. Acquired Immunity

Passive immunity antibody-mediated protection in neonates provided by

placental or colostral transfer of pathogen specific antibodies

can protect offspring weeks - months

interfere with the development of active immunity following vaccination

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Acquired Immunity Cont’d

Active Immunity

Humoral (Antibodies)- bacterins and subunit vaccines

typically induce this type of immune response

Cell mediated - critical arm of immune response for

clearance of intracellular pathogens

Components include T-cells and T-cell activated phagocytic

cells