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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
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