Introduction to the Microbial World 1

Professor A G Duse

Chief Specialist and Head (Chair): CMID

NHLS and Wits School of Pathology

Room 3T11, Level 3, WMS

Tel #: 489 8510

Lecture Objectives:

1. Briefly describe the evolutionary relationships of living organisms

2. List the main distinguishing features of prokaryotic versus eukaryotic cells

3. Provide a simple classification of the different forms of microbial life

4. Understanding the underlying principles of microbial taxonomy

5. Understand the concept of microbial morphology and its importance in clinical microbiology

6. Draw an annotated diagram of a typical bacterial cell

Molecular Phylogeny:

• Cyanobacteria (~3.6 billion years old) earliest evidence of unicellular life• Molecular phylogeny: tool that enables us to understand the complexity of

life & recognize the relationships of living forms• Determined by comparing difference in homologous genes encoding

ribosomal RNA (16S r RNA genes from PROKARYOTIC cells & 18S rRNA from eukaryotic cells)

• Computer analysis of rRNA gene sequences => life has evolved along 3 major lineages:1. Bacteria &2. Archae – not clinically significant (solely microbial & composed of only unicellular

organisms), and3. Eukaryotes (more complex, multi-cellular organisms)

• Hence, prokaryotes & eukaryotes; both ancient & derived from a common UNIVERSAL ancestor!

• Homo sapiens & other species either live in harmony (colonization) or are diseased (infected with manifest symptoms and signs to cause PATHOLOGY) with both prokaryotic and eukaryotic organisms!

Differences Between Prokaryotic and Eukaryotic Cells:

PROKARYOTES nucleoid no nuclear membrane haploid chromosome no mitotic division (replicate

by binary fission) no mitochondria no Golgi apparatus,

microtubules 70S ribosomes motile by flagella

EUKARYOTES nucleus present diploid mitotic division

present

present 80S ribosomes more complex

Classification of Microbes:

DNA or

RNA

Cocci

Bacilli

Spiral

Protozoa

Trematodes

Cestodes

Nematodes

Yeasts

Dimorphic

Moulds

Micobial World

Viruses Bacteria Fungi Parasites

Nomenclature (Taxonomy) of Bacteria:

FAMILY:

The Enterobacteriaceae

GENUS:

Salmonella Escherichia

SPECIES:typhi (S typhi OR S typhi)

coli(E coli OR E coli)

Binomial classification: Genus and species

Bacterial Morphology:

SIZE: micronsSHAPE: ARRANGEMENT

e.g. cocci in chains

e.g. cocci in clusters

ULTRASTRUCTURAL FEATURES: capsules, flagella, fimbriae / pili; spores

Bacterial Cell Structure:

Introduction to the Microbial World 2

Professor A G Duse

Chief Specialist and Head (Chair): CMID

NHLS and Wits School of Pathology

Room 3T11, Level 3, WMS

Tel #: 489 8510

Lecture Objectives:

• Using a bacterial cell as an example, describe the ultra-structural features of bacteria & discuss both their laboratory & clinical relevance

• Discuss the role of bacterial cell components in disease causation (pathogenesis)

• Classify bacteria according to their morphology, aero-tolerance, and staining reactions with particular emphasis on Gram and acid-fast stains

Bacterial Cell Components:

Bacterial Cell Components:

Capsule (mostly polysaccharide): antiphagocytic; antigenic/ immunogenic

Flagella (proteinaceous): locomotion; antigenic; ? Immune evasion Fimbriae/pili: adherence Cell wall (cytoskeleton = peptidoglycan): rigidity & shape;

protection against osmotic pressure - prevention of lysis; antigenic Cytoplasmic membrane: cell respiration; cell precursor synthesis Intracytoplasmic inclusions; DNA; ribosomes Spores: protect species of genera Bacillus and Clostridium from

unfavourable conditions

Construction of the Cell Walls of Gram-positive and Gram-negative Bacteria:

LIPOPOLYSACCHARIDE (ENDOTOXIN)

Bacterial Morphology:

STAINING REACTIONS:Gram staining:

Gram-positive (dark-blue/purple)

Gram-negative (pink)

Acid-fast:• stains poorly with Gram stain e.g. Mycobacterium

tuberculosis

• Ziehl-Neelsen stain; Kinyoun stain; auramine stain

Morphology-based bacterial classification:

Man versus Microbes:

Professor A G Duse

Chief Specialist and Head (Chair): CMID

NHLS and Wits School of Pathology

Room 3T11, Level 3, WMS

Tel #: 489 8510

Objectives:

• Understand the concept of infectious disease causation• Discuss the interactions between hosts, microbes and the

environment• Describe the concepts of true virulence versus opportunism• List, and using appropriate examples, discuss the 7 major

challenges that a microbe must overcome to cause infection• Illustrate all of the above by briefly discussing the recent

outbreak caused by Rift Valley Fever virus in Kenya (Dec 2006-Jan 2007)

Definitions:

• Pathogenesis

• Pathogens: “true”/”primary” vs. “opportunistic” vs. “colonizers”

• Carrier state

• Virulence: ID 50, LD 50; other

HOST

MICROBE ENVIRONMENTFOCUS OF TODAY’S LECTURE

X-REF IMMUNOLOGY: INNATE/NON-SPECIFIC DEFENCE MECHANISMS

Pathogenic Bacteria:The Seven Challenges

• Maintain a reservoir– Human, animal, environmental

• Gain access to a new host– Portal of entry; mode of transmission

• Adherence: non-specific & specific mechanisms• Establishing infection: evading host defences (X-ref: Immunology)• Mechanisms of disease causation

– Adherence; toxins (exotoxins, endotoxin); hypersensitivity/autoimmune reactions (X-ref: Immunology)

• Exiting from one host, entering another

Portals Of Entry:

• Respiratory tract

• Gastrointestinal tract

• Genitourinary tract

• Skin and mucous membranes

Modes Of Transmission:

• By respiratory droplets; droplet nuclei• Airborne (other than above)• Faecal-oral• By direct body contact• By fomites• Parenteral• By arthropod vectors

Remember:

• Adherence; + / -

• Invasion; +/ -

• Toxin production– Differences between exotoxins & endotoxins; + / -

• Hypersensitivity reactions

Pathogenic Bacteria:The Seven Challenges

• Maintain a reservoir– Human, animal, environmental

• Gain access to a new host– Portal of entry; mode of transmission

• Adherence: non-specific & specific mechanisms• Establishing infection: evading host defences (X-ref: Immunology)• Mechanisms of disease causation

– Adherence; toxins (exotoxins, endotoxin); hypersensitivity/autoimmune reactions (X-ref: Immunology)

• Exiting from one host, entering another

Rift Valley Fever:Rift Valley Fever:

RVF Kenya Dec 06 - Jan 06:

Natural cycle of RVFV:Natural cycle of RVFV:

Infected flood water Aedes & Culex sp. oviposit

Eggs with virus dormant

Heavy rains flood

Infectious mosquitoes hatch

Mosquitoes feed on livestock and occasionally humans

Amplification in vertebrate host

Ocular RVFV:Ocular RVFV:

Al-Hazmi et al., CID 2003; 36:245

‘‘Macular scarring’Macular scarring’

Hemorrhagic RVF:Hemorrhagic RVF:

Epizootic cycle of RVFV:Epizootic cycle of RVFV:

Livestock (mainly sheep, cattle, goat) infected by vector

Incubation followed by viremia and illness

Feeding by uninfected vectors

Extrinsic incubation

Newly infectious vector

Slaughter, abortion, or necropsy

Aerosol or contact infectionof humans

• Many mosquitoes world-wide can be vectors in the lab and in the field at blood meal virus concentrations found in sheep, cattle, humans

• We know it can cause epidemics when vertebrates that develop high viremias are present (sheep, cattle, goat)

• Mortality in domestic livestock and subsequent trade interruption have severe economic impact

• Humans may visit endemic/epidemic areas and return to their homes within an incubation period (e.g., tourist with retinopathy)

• Both arthropod and direct transmission from blood efficient (concern: bioterrorism and aerosols)

• Established epidemics in new territory: Egypt, Saudi, Yemen

• Many mosquitoes world-wide can be vectors in the lab and in the field at blood meal virus concentrations found in sheep, cattle, humans

• We know it can cause epidemics when vertebrates that develop high viremias are present (sheep, cattle, goat)

• Mortality in domestic livestock and subsequent trade interruption have severe economic impact

• Humans may visit endemic/epidemic areas and return to their homes within an incubation period (e.g., tourist with retinopathy)

• Both arthropod and direct transmission from blood efficient (concern: bioterrorism and aerosols)

• Established epidemics in new territory: Egypt, Saudi, Yemen

RVFV – a worldwide concernRVFV – a worldwide concern

THANK YOU !