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Outline of Lectures
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Review of the work on contagious disease by Koch.
Epidemiology and Public Health.
Microbial interactions with higher animals.
Examination of the normal flora of animals.
Entry of the pathogen into the host. Colonisation
and growth.
Transmission of pathogens. Bacterial respiratory
infections and sexually transmitted bacterial
diseases.
Insect transmitted diseases.
Food-borne and water-borne bacterial diseases.
BS104 Lecture 20
Microbial interactions with higher
animals. Examination of the “normal
flora” of animals.
Human tongue bacterial flora
Microorganisms and Humans
• Normal microbial flora: Microbes that are usually
found growing on or within the body of humans.
• Everyday the human body is constantly being exposed
to new microbes.
• Colonisation: growth of microorganisms after
accessing the host cells/tissues.
• Humans are colonised by bacteria at birth.
• The human gut has been described as a bacterial
culture vessel.
• Normal microbial flora is important for general wellbeing. Most microbes are benign (cause no problems)
and some even contribute to health.
• However, bacteria sometimes do invade (pathogens).
Pathogens
•A host is an organism that harbours a parasite,
another organism that lives in or on the host and
causes disease.
•A microbial parasite is called a pathogen.
Outcome of a human:microbe interaction
• Pathogenicity
- Ability of a parasite to inflict damage on the host
- Pathogenicity varies amongst different pathogens.
- As does the host resistance or susceptibility.
– Virulence is a measure of pathogenicity.
– Changing conditions in the pathogen, host and the
environment make the host-microbe interaction
dynamic.
Infection and Disease
•Infection is any situation leading to the establishment
of microbial growth with or without causing damage to
the host.
•Microorganisms cause disease when their existence in
the host leads to damage or injury to the host
resulting in host function impairment.
•However, not all infections result in disease. Thus
infection with the normal flora does not usually cause
disease.
Infection and Disease
• Disease might occur if the
individual is unhealthy e.g.
an immuno-compromised
individual (e.g. Acquired
immunodeficiency
syndrome [AIDS] patient
or cancer patient)
• An opportunistic pathogen
causes disease in the
absence of the normal
host resistance.
MRSA patient
Animals as microbial hosts
• Rich in nutrients
• Constant suitable
temperature
• pH
• Osmotic pressure
• Various environments
(skin, gut, nose)
• Dry conditions (skin):
Staphylococcus aureus.
• Highly oxygenated (lung):
Mycobacterium
tuberculosis (an obligate
aerobe).
• Anaerobic (colon):
Clostridium genus
(obligate anaerobes)
Therefore, animals provide a favourable environment for the growth of
many microbes.
Microbial infections
Microbial
cells
Mucus
Epithelial
cell
Brock: Figure 28.1
• Mucous membranes (mouth, pharynx, urogenital,
gastrointestinal etc).
• Epithelial cells and mucus (soluble glycoproteins) layer.
• Main steps of infection (a) – association, (b) adhesion, (c)
invasion into submucosal epithelial cells.
Beneficial Microbial Interactions
with Humans
• From birth onwards exposure of
humans to microbes is constant.
• Normal flora: billions of
microorganisms grow within or on us.
Normal flora of skin
• Adult human has ~2m2 of
skin
• Skin surface not generally
favourable for bacterial
growth as it is too dry.
• Sweat glands are
favourable.
• Most skin microorganisms
are associated with
apocrine sweat glands
(under arms, genital
regions, the umbilicus).
• Hair follicles are also an
attractive environment for
microorganisms.
Brock: Figure 28.2
Normal flora of skin
• Over 180 species of bacteria and several species of fungi have
been found to inhabit the skin.
• Gram-positive bacteria including species of Streptococcus,
Staphylococcus, Corynebacterium and Propionibacterium
(Propionibacterium acnes – causal agent of acne) are the most
common and stable microorganisms that reside on the skin.
• Skin can be also be contaminated with gram-negative bacteria
e.g. enterics by faecal contamination but they will not grow.
• However, the gram-negative rod Acinetobacter is an exception.
• Malassezia spp., Pityrosporum ovale and Candida are some of
the fungal species that inhabit the skin.
• The weather, the age of the host and personal hygiene
influence the variety and the density of the microflora.
Some common skin microbes
• Mainly gram positive as
cell walls are stronger
and less prone to drying.
• Common microbes
include:
– Staphylococcus species.
– Corynebacterium.
– Propionibacterium acnes
(acne).
– Yeast.
Cell walls
C.M. = cell membrane
Flora of the Oral Cavity
• Saliva contains very few nutrients in low
concentrations
• Contains antibacterial compounds such as:
– lysozyme (breaks down peptidoglycan in
bacterial cell wall, weakening the wall and
causing cell lysis) and,
– Lactoperoxidase, an enzyme found in both milk
and saliva (kills bacteria and generates
antibacterial singlet oxygen in the process).
• The problem is food (rich in nutrients) especially refined food with a high sugar
content.
Flora of teeth
(Formation of dental plaque)
• Acidic glycoproteins from
saliva form a thin organic film.
• Bacterial microcolonies attach
to this film on tooth surface.
• Mainly Streptococcus species
(S. sanguis, S. sobrinus, S.
mutans and S. mitis) colonise
the glycoprotein film.
• Dental plaque forms upon
extensive growth of these
bacteria, leading to further
colonization (Fusobacterium
species, Borrelia species).
• Build up of plaque leads to
anaerobic environment.
Brock: Figure 28.3
Microbial microcolonies
Bacterial growth on a model tooth – note arrows showing matrix that
glue bacteria together and to surface. Brock – Figure 28.4.
Dental Caries (cavities)
• Accumulation of dental
plaque leads to
production of organic
acids that cause
decalcification of enamel,
resulting in dental caries
(tooth decay) – an
infectious disease.
• As sugars in diets high in
sucrose are fermented to
lactic acid (S. mutans and
S. sobrinus), proteolysis
of enamel matrix then
occurs and dental caries
are promoted.
• Decalcification prevented
by fluoride treatment.
Streptococcus mutans
• Produces a dextran
polysaccharide adhesive from
sucrose (table sugar).
• 80-90% of the US and
Western Europe population
have S. mutans colonization.
• No sugar in diet: no plaque.
• Susceptibility varies amongst
individuals.
• S. mutans is common in the
US and Europe (teeth decay)
but absent in plaque of
Tanzanian children (no decay).
• Microorganisms can also
infect and cause the
inflammation of the gum
tissues (gingivitis).
Sticky dextran holds cells together
Flora of the Gastrointestinal Tract
• Human gastrointestinal (GI) tract consists of the stomach, small
intestines and large intestines (see next slide).
• Responsible for digestion of food, absorption of nutrients and
synthesis of nutrients by the residing microbial flora.
• About 1013 to 1014 microbial cells are present in the entire GI
tract.
• Stomach fluids are highly acidic (pH 2) – therefore the stomach
acts as a chemical barrier for GI tract.
• Studies using 16S rRNA sequences from human stomach biopsies
have revealed that bacteria do populate this hostile environment,
albeit at a lower number than the small and large intestines .
• Individuals have very different bacterial populations in their
stomach, but all contain gram-positive bacteria (Proteobacteria,
Bacteroidetes, Actinobacteria and Fusobacteria)
• Also, remember Helicobacter pylori and stomach ulcers - the work
of Barry J. Marshall and J. Robin Warren).
Flora of the Gastrointestinal Tract
Major bacteria present
Esophagus
Organ
Esophagus
Prevotella
Streptococcus
Veillonella
Helicobacter
Gram-positive bacteria
Proteobacteria
Bacteroidetes
Actinobacteria
Fusobacteria
Duodenum
Enterococci
Lactobacilli
Jejunum
Bacteroides
Bifidobacterium
Clostridium
Enterobacteria
Enterococcus
Escherichia
Eubacterium
Gram-positive bacteria
Klebsiella
Lactobacillus
Methanobrevibacter
Peptococcus
Peptostreptococcus
Proteus
Ruminococcus
Staphylococcus
Streptococcus
Major physiological processes
Stomach
Secretion of acid (HCl)
Digestion of macromolecules
pH 2
Small
intestine
Continued digestion
Absorption of monosaccharides,
amino acids, fatty acids, water
pH 4–5
Large
intestine
Absorption of bile acids,
vitamin B12
pH 7
Ileum
Colon
Anus
Brock: Figure 28.8
Intestinal Tract
• Small intestine: Separated into two parts: the
duodenum and the ileum connected with the jejunum.
Duodenum is also acidic but pH increases towards ileum
and bacterial numbers increase reaching 105-107
cells/gram digestive material in the lower ileum.
• Large intestine: Enormous bacterial numbers of 10101011 obligate anaerobes cells/gram. (See previous figure
for flora). Some facultative aerobes such as E. coli are
present (in smaller numbers than other bacteria).
Oxygen is consumed by these and the result is a
strictly anaerobic environment. This creates an
environment for growth of obligate anaerobes,
including species of Clostridium.
Normal Microbial GI Tract Flora
• Diet influences the microbial populations in
the GI tract.
• Vegeterians show higher numbers of coliforms
and lactic acid bacteria than do individuals
with high consumption of meat.
• Many microorganisms cannot colonise the
stomach and the duodenum due to the highly
acidic (~pH 2) environment.
Normal Microbial GI Tract Flora
• Functions and Products of Intestinal Flora
– Various compounds are produced through
biochemical/metabolic processes, by the intestinal
microorganisms.
– Diet and the composition of the intestinal flora are
influencing factors in the production of the type
and amount of the compounds.
• Compounds produced include
– Vitamins B12 and K (not synthesised by
humans , but by intestinal microflora. They
are then absorbed from the gut).
– Gas (CO2, CH4 and H2)
Contributions of intestinal
microbes
Changing the normal flora
•Oral antibiotics interfere with growth of normal flora –
signalled by loose faeces or diarrhoea.
•Opportunistic bacteria such as antibiotic-resistant
Staphylococcus, Clostridium difficile or the yeast
Candida albicans can become established in the absence
of normal flora.
•After antibiotic treatment, the normal intestinal flora
is re-established relatively quickly.
•Probiotics also help with the re-establishment of the
desired species of bacteria.
Respiratory Tract
• Microorganisms are inhaled but
most get trapped in nasal cavity.
• Staphylococcus and Streptococcus
are normal flora. Potential
pathogens such as Staphylococcus.
aureus and Streptococcus
pneumoniae may also be present.
• Immune system and competition by
other bacteria keep their growth in
check.
• Lower respiratory tract is sterile.
• As breathed air velocity slows,
bacteria stick to walls of airways.
• A lining of ciliated cells also help to
eliminate bacteria from lower
tract.
Sinuses
Upper
respiratory
tract
Nasopharynx
Pharynx
Oral cavity
Larynx
Lower
respiratory
tract
Trachea
Bronchi
Lungs
Brock: Figure 28.10
Urogenital Tract
• Bladder itself is typically
sterile (M&F).
• Urethra is colonized by
Escherichia coli and
Proteus mirabilis.
• Sometimes these are
opportunistic pathogens
(pH changes) and cause
UTI, especially in women.
• Vagina (weakly acidic in
adult) contains
Lactobacillus acidophilus
that ferments glycogen and
lowers pH.
• Before puberty (and after
menopause) glycogen is not
present and so pH
increases.
• Flora thus changes
according to maturity.
Lactobacillus acidophillus (Gram-stain)
Brock: Figure 28.11.
Points to consider
• Where in the body are bacteria found?
• What type of bacteria are they and how are they
suited to that particular environment?
• What parts of the body must be sterile?
• Where in the body are bacterial numbers the
highest?
EXPOSURE
to pathogens
Pathogenesis
Mechanisms
ADHERENCE
to skin or mucosa
Further
exposure
at local
sites
INVASION
through epithelium
Further
exposure
COLONIZATION
and
GROWTH
Production of virulence factors
TOXICITY:
toxin effects are local
or systemic
INVASIVENESS:
further growth at original
site and distant sites
TISSUE DAMAGE, DISEASE
Further reading
• Brock 12th edition Chapter 28
• Brock 11th edition Chapter 21