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Nutrition, Growth, and Metabolism. In Medical Microbiology, microbial cultivation is required for the following purposes In Medicine 1-Diagnosis of most infectious diseases 2-Selection of drug of choice ( antibiotics ) for treatment of infection In other fields

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In Medical Microbiology, microbial cultivation is required for the following purposes

    • In Medicine
      • 1-Diagnosis of most infectious diseases
      • 2-Selection of drug of choice ( antibiotics ) for treatment of infection
    • In other fields
      • 3-Preparation of Vaccine
      • 4-Research tool in molecular genetics
microbial growth requirements
Microbial growth requirements
  • What are the chemical growth factors required for isolation of microbes in vitro?
    • 1-Carbon
      • Organic source Glucose
      • Inorganic source CO2
    • 2-Nitrogen
      • Organic source Protein
      • Inorganic source Atmospheric nitrogen
    • Autotrophic microbes using inorganic carbon and nitrogen
    • Heterotrophic microbes using organic carbon and nitrogen
    • Other chemical requirements
      • 3- Hydrogen, Oxygen, Phosphorus, Carbon, Nitrogen, Ions, and Sulfur
physical growth factors
Physical growth factors
  • 1- pH and Buffer requirements
    • Pathogenic bacteria grow best at neutral pH which is typically between pH (6.8 to 7.4)
    • Acidophilic bacterium is able to survive with acidic pH
      • Example: Helicobacter pylori
    • On the other extreme, bacteria that prefer alkaline conditions are known as alkaliphiles
      • Example: Vibrio cholerae
    • Fungi such as yeasts and molds; grow best at acidic conditions ( pH 5 )

2- Salts concentration

    • A specific concentration of NaCl is required for microbial growth in vitro
    • It is equal to normal saline salts concentration (0.9% NaCl)
    • Halophilic bacteria resist high salt concentration

3- Temperature requirements

    • Mesophiles
      • grow at optimum of 37 ᵒC
      • human body temperature
        • Pathogens
        • Opportunists
    • Pyschrophile
      • close to freezing
    • Thermophile
      • close to boiling
4 gaseous requirements and humidity
4- Gaseous requirements and Humidity
  • According to oxygen requirements, microbes can be divided into the following groups
  • 1- Strict or Obligate Aerobe
    • O2 must be present to grow
    • Example: Mycobacterium sp.
  • 2- Strict or Obligate Anaerobes
    • Oxygen must be absent
      • Killed by oxygen
    • Example: Clostridium sp.

3- Facultative anaerobes

    • Adaptable organisms that use oxygen when present but can switch to anaerobic pathways in its absence
    • Survive in the presence of O2
      • Example: Escherichia coli
  • 4- Microaerophilic
    • Only use low concentrations of oxygen ( around 5%)
      • Example : Helicobacter pylori
    • Grow in low oxygen
    • Killed in high oxygen
the bacterial growth curve
The Bacterial Growth Curve
  • Bacterial growth is the division of one bacterium into two daughter cells in a process called binary fission
  • Bacterial growth curve represents the relationship between microbial quantity and time of incubation
  • During Lag phase, bacteria adapt themselves to growth conditions (number of dividing cell is zero)
  • Exponential phase ( Log phase): is a period characterized by Rapid cell doubling
  • Doubling time can be as short as twenty minutes or as long as several days

During stationary phase, the growth rate slows as a result of nutrient depletion and accumulation of toxic products

  • This phase is a constant value as the rate of bacterial growth is equal to the rate of bacterial death
  • At death phase, bacteria run out of nutrients and die
microbial metabolism
Microbial Metabolism
  • According to biochemical pathway used in energy production, bacterial metabolism can be categorized into three types:
  • 1-Aerobic Respiration
    • Molecular oxygen serves as the final electron acceptor
    • 38 ATP molecules will be produced by oxidation of one glucose molecule
    • Used by obligatory aerobic bacteria for energy production
      • such as: Mycobacterium sp.

2-Anaerobic Respiration

    • Inorganic sulfate or nitrate act as the final electron acceptor
    • 38 ATP molecules will be produced by catabolism of one glucose molecule
    • Used by obligatory anaerobic bacteria such as: Clostridium sp.
  • 3-Fermentation
    • Lactic acid ( produced by bacteria) or ethanol ( produced by yeast) serves as final electron acceptor
    • Only 2 ATP molecules will be produced by fermentation of one glucose molecule
    • Used by facultative anaerobic bacteria such as : E. coli
microbial genetics
Microbial Genetics
  • Prokaryotic Genome
    • Most prokaryotic genes are carried on the bacterial chromosome, a single circle of DNA
    • Many bacteria contain additional genes on plasmids
    • Plasmid is an extra-circular supercoiled DNA that carry some important genes such as the antibiotics resistance genes
    • Both bacteria chromosome and plasmid are called replicons

Genetics is the study of inheritance and variation

  • Genetic information encoded in DNA
  • Function of genetic material
    • 1- Replication of the genome
    • 2- Expression of DNA to mRNA then to protein
  • Genotype
    • the complete set of genetic determinants of an organism
  • Phenotype
    • expression of specific genetic material
bacterial dna
Bacterial DNA
  • 2 types of DNA in bacteria
    • Chromosomal
    • Extra-chromosomal (plasmid)
  • Extrachromosomal DNA
  • Found in most species of bacteria.
  • Govern their own replication
  • Genetic exchange, amplify genes
  • Transfer by conjugation
  • Code for resistance to antibiotics & toxins
gene transfer
Gene Transfer
  • Transfer of DNA among prokaryotes is widespread between different strains of same bacterial species
  • Mechanisms of Gene Transfer
    • 1- Conjugation
    • 2- Transduction (is a phage-mediated genetic transfer)
    • 3- Transformation
bacterial conjugation
Bacterial conjugation
  • Conjugation: is a mechanism of gene transfer by which plasmids will be transferred from one bacterial cell to another by a mean of Sex pili
  • Hospital-dwelling bacteria resist antibiotics due to conjugation
  • Transduction is the process by which DNA is transferred from one bacterium to another by a virus
  • It also refers to the process whereby foreign DNA is introduced into another cell via a viral vector
bacterial transformation
Bacterial transformation
  • A stable genetic change brought about by the uptake of naked DNA and competence refers to the state of being able to take up exogenous DNA from the environment
  • There are two forms of transformation and competence: natural and artificial
the microbial virulence factors
The Microbial Virulence factors
  • Virulence factors are external cellular structures, enzymes, and toxins that enhance microbial pathogenicity
  • In general, the most important virulence factors are:
  • 1-Microbial capsule
    • Microbe resist host acidic environment (stomach gastric acid)
    • Microbe resist host proteolyticenzyme
      • (Present in Saliva, and stomach)
    • Microbe resist phagocytosis

2-Fimbriae or Pili

    • Microbial adhesion to the host cell surface
    • Adhesion could be also enhanced by receptor-antigen interaction
  • 3-Microbial Enzymes
    • Collagenase enzyme
      • enhances microbial invasion; due to degradation of extracellular matrix components
    • Urease
      • Neutralization of acidic pH ( urine, stomach)
    • Coagulase
      • Catalase have different functions

4-Bacterial Toxins

    • A-Exotoxins
      • Well known poisonous substances.
      • Chemical nature
        • Proteins (two polypeptide components)
      • Almost all are Heat-labile at 60 ˚C
    • Intracellular toxin fraction could
      • 1-Inhibit cellular protein biosynthesis
      • 2-Cause ionic imbalance and loss of water
      • 3-Inhibit the release of neurotransmitters


    • Chemical nature
      • Lipopolysaccharide, the component of Gram’s negative bacterial outer membrane
    • Heat-Stable at 100 ˚C
  • 5-The microbial Hemolysin
    • Degradation of RBCs, Hemoglobin and NADH will be released
  • 6-The microbial Haemagglutinine and Coagulase enzyme
    • Agglutination of RBCs; the microbe escapes Humoral immunity

7-The microbial Beta-Lactamases

    • Some microbes have ability to resist antibiotics due to production of Beta-Lactamase enzymes
    • Some strains of Staphylococcican hydrolyze the Beta-Lactam ring shown

In the absence of β-lactam antibiotics, the bacterial cell wall plays an important role in bacterial reproduction.


Adding β-lactam antibiotics to the cell medium while bacteria are dividing will cause them to shed their cell walls and fail to divide, forming large, fragile spheroplasts.