Unit 67421 Lecture 3 Introduction to the bacteria

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Unit 67421 Lecture 3Introduction to the bacteria

• Lecture aims
• To become aware of bacterial diversity
• To understand the nature of bacteria
• To understand how Bacteria grow
• To understand how bacteria can be isolated
• References Black Chp 4 p71-85 Chp 6p137-160

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Bacterial diversity

• Bacteria are very adaptable and fill many niches
• Eg animals (commensals, pathogens)
• Skin, gut, mucous membranes
• soil (free living) and spores
• Symbiosis
• (eg Rhizobium sp legumes -gtN2fix)

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Bacterial diversity contin.

• Some species have become very specialized
• Eg the obligate anaerobes such as Clostridia sp
• Others have preferred conditions but can tolerate
  less favorable ones
• Eg Escherichia coli prefers to grow with oxygen
  present but can grow in its absence

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The nature of bacteria

• Shape
• Bacteria may have many shapes and sizes
• Some (most) have a cell wall which contains a
  special molecule (peptidoglycan)
• Bacteria without a cell wall do exist- an
  important character is their resistance to
  penicillins! Why?
• Conformation
• Conformation can be single, clusters, diploid,
  tetrad, chains- what influences this?

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Bacterial shapes Examples include

• Rods
• eg Bacillus anthracis
• Pleomorphic rods
• eg. Escherichia coli

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Bacterial shapes contin...

• Cocci (circular)
• Eg 1. Streptococcus sp
• Eg 2. Staphylococcus sp

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Bacterial shapes contin...

• Spirals
• eg. Treponema pallidum
• (cause of syphilis)
• Seagull shaped
• eg Vibrio cholera
• (cause of cholera)

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Basic external structuresOften important in
making vaccines-why?

• Pili hair like structures used for attachment
  and mating (sex pili)
• Flagella whip like structure for movement
• Capsules sticky sugary base, can be used for
  attachment and to avoid immune system

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Basic internal structures

• The Cell wall
• A major component is glucose based polymer called
  peptidoglycan
• Consists of cross linked molecules of
• N-acetylglucoseamine
• N-acetylmuramic acid

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Cell wall contin...

• Petidoglycan content differs between Gve and
  G-e bacteria
• Gram bacteria have a dominant thick
  peptidoglycan layer (with some linkage changes as
  well)

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Cell wall contin...

• While G-e bacteria have a more complex cell wall
  layer with inner and outer cell membranes (LPS)
  thinner layer of pepidoglycan

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Bacterial growth1. Physical requirements

• Temperature preferences
• Note tends to be a range with growth most rapid
  at preferred temperature
• Bacterial growth measured by numbers of
  bacteria/time

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Bacterial growth contin...

• Bacteria defined according to temp. preferences
• Psychrophyles (0-20oC)
• eg Listeria monocytogenes
• Mesophyles (20-40oC)
• eg E.coli (most endogenous bacteria including
  pathogens)
• Thermophyles (gt45oC)
• eg Thermophilus aquaticus 90o

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Growth requirements2. Oxygen needs

• Obligate aerobe eg Pseudomonas aeruginosa
• Obligate anaerobe eg Clostridium botulinum
• Facultative anaerobe eg E.coli
• Microaerophilic eg Haemophilus influenzae
• Aerotolerant eg Micrococcus sp

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Oxygen preferences contin...

• Figure illustrates growth of various bacteria in
  liquid culture according to their tolerance/need
  of oxygen

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Bacterial isolation (labs)

• Need to supply nutrients physical chemical
• Some very finicky (fastidious)
• Can take a long time (TB gt6 weeks)
• Often need special media (Choc agar)
• Supply correct temp and oxygen needs
• Can be enhanced by using special media

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Bacterial isolation contin...

• Selective media
• Selective differential (MSA, McConkey)
• Mannitol Salt Agar (Staphylococcal)
• High salt selects for staphylococci
• Mannitol separates S.aureus (yellow) Others
  (pink)
• McConkey (enteric bacteria)
• Bile salts inhibit other than enteric(gut)
  bacteria)
• Enrichment (selenite broth BHI)
• Inhibits non target bacteria, allows small
  numbers of target to grow and be isolated

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Bacterial growth in a closed cultureeg. in a
broth

• Lag adaptive(slow)
• Log fastest possible under conditions (newgtthan
  dying)
• Stationary newdying
• Deathdyinggtnew