OVERVIEW

• Growth refers to the increase in number of bacterial cells, which occurs via binary fission.
• Ultimately, growth can produce a colony of millions of bacterial cells.
• The time it takes for takes for the cell population to double is the “generation time”.
  — This time varies by species, and is moderated by environmental factors such as pH, nutrient availability, temperature, etc.
  — For example, the generation time for Staphylococcus aureus grown in heart infusion broth is about 30 minutes.
• Bacteria are haploid
  — E. coli, which we use in our diagram, have chromosomal DNA organized into circular, double-stranded structures.
• Pathogenicity islands refer to the distinct regions of some bacterial chromosomes that code for virulence factors; these islands are absent in non-virulent strains.
• Extrachromosomal genetic elements* may also be present.
  For example, plasmids and bacteriophages may engage in horizontal DNA transfer.
• Quorum sensing is a type of bacterial communication that arises when cell population density is high.
  — Quorum sensing is mediated by autoinducers, which are molecules released by bacterial cells.
  — As cell density increases, so does autoinducer concentration.
  — In different species, quorum sensing moderates virulence factor secretion, biofilm production, sporulation, and other behaviors.
  — For example, consider the bacteria Staphylococcus aureus, which produces autoinducer peptides (AIPs).
  — When the density of S. aureus increases, so does the concentration of autoinducer peptides. In turn, this induces bacterial release of virulence factors, including several toxins.
  Furthermore, the cells are stimulated to release additional AIPs, thus creating a positive feedback loop.
  Because quorum sensing facilitates some of the harmful effects of acute S. aureus infection, researchers are investigating treatments that prohibit or moderate this type of cellular communication.

BACTERIAL GROWTH CURVE

Tracks the stages of cell population growth.
— We’ll plot time along the x-axis, and the log number of cells along the y-axis.

• Lag stage
  — Bacterial cells engage in metabolic activity but not cell division; during this stage, the bacteria acclimate to the growth conditions.
• Logarithmic
  — Aka, exponential stage.
  — Rapid cell division occurs.
  — Beta-lactam antibiotics, such as penicillin are effective during this period, because they interfere with cell wall production.
• Stationary stage
  — The curve plateaus during the stationary phase because proliferation and cell death are in balance; this steady state is reached when nutrients are running low and/or toxin levels are elevated.
• Death stage
  — Finally, the number of bacteria declines.
  — However, some bacteria may remain viable during this stage.

Important points regarding the growth cycle:

• Growth requirements include carbon, nitrogen, energy sources, water, and ions; though specific requirements vary by species.
  — Iron is so important for growth that some bacteria can secrete siderophores that “steal” iron from the host.
• Obligate anaerobes cannot grow in the presence of oxygen.
  — Clostridium is an example.
• Obligate aerobes can only grow in the presence of oxygen.
  — Mycobacterium tuberculosis
• Facultative anaerobes can grow with or without oxygen.
  — Most common; Staphylococcus, which contributes to the normal flora of the nares, is an example of this.
• Obligate intracellular pathogens they can only grow within living cells because they rely on ATP derived from the host
  — Chlamydia

CHROMOSOME REPLICATION IN E. COLI

• Occurs via binary fission – chromosome replication triggers initiation of cell division.
• We illustrate this in Escherichia coli; be aware that not all bacteria replicate in this exact manner.

First drawing

• Single circular chromosome with inner and outer strands.
  — In reality, bacterial DNA is arranged in loops. Recall that there is no distinct nucleus, as in eukaryotic cells; instead, DNA lies in the nucleoid region.
• Origin of replication is marked by oriC; this is where the replication initiator proteins bind.
• Terminus is located opposite oriC; this is the region where DNA replication terminates.

Second drawing
After DNA replication begins:

• Outer and inner parental strands begin to separate in bidirectional replication.
• Replication “bubble,” is the space between the strands.
• Two y-shaped replication forks, one on each side of the bubble.
• DNA helicases separate the parental DNA strands in short segments, thus creating the replication forks; if the two strands separated all at once, excessive DNA damage could occur.
• Developing daughter strands: each one begins at the origin of its parental strand and grows towards its terminal end.
  — These complementary daughter strands are synthesized by the replisome, which comprises DNA polymerase III and other components.

Third drawing

• The parental and growing daughter strands are further along in the replication process.
  — The parental strands have further separated from each other as the daughter strands grow towards the terminal region.

Fourth drawing

• Finally, the chromosomes separate after the daughter strands are complete.
• DNA replication is semiconservative:
  — Each new chromosome comprises one strand of DNA from the parental chromosome and one complementary daughter strand.

BASIC COMPONENTS OF A BACTERIAL CELL

• Be aware that different strains of bacteria have special anatomical and physiological traits, which we address elsewhere as these features relate to infectious disease.
• Cell wall
• Plasmic/cytoplasmic membrane is deep to the cell wall.
  – It has an area of infolding. The membrane comprises a lipid bilayer that serves multiple functions, including transport of molecules into the cell, secretion of toxins and enzymes, and energy generation.
• Cytoplasm
  – Ribosomes are sites of protein synthesis.
• Nucleoid is the region with bacterial DNA; there is no nuclear membrane.
• Outer capsule, which most often comprises gelatinous polysaccharides.
  – Variable; When present, the capsule contributes to the serologic type and enhances virulence.
• Pili, aka, fimbriae, which are short filaments found primarily on gram-negative strains.
  – Variable; When present, they participate in bacterial attachment to host cells.
  – The sex pilus attaches donor and recipient bacteria during conjugation.
• Flagella
  – Variable; when present, propels the cell; the number of flagella varies.

STAINING

• Most bacteria associated with infectious disease can be categorized as gram-positive or gram-negative, which refers to whether they retain crystal violet stain, and depends on the composition of their cell walls.

Gram-positive

• In a microscopic sample, we can see the bright purple stain – think Purple Positive.
• Cell wall has thick layer of peptidoglyclan (aka, murein and mucopeptide)
  – It comprises a network of sugars and amino acids, and is the target of some antibiotic drugs.
  – We indicate two acids that are present in the cell wall: teichoic acids, which attach to the peptidoglycan, and, lipoteichoic acid, which are attached to the cytoplasmic membrane.
  – These surface acids contribute to the cell wall structure and charge.

Gram-negative

• In a microscopic sample, gram negative bacteria appear reddish-pink.
• They have a thin peptidoglycan layer, which is covered by the outer membrane.
• The outer membrane is unique to gram negative bacteria and comprises lipopolysaccharide, aka, endotoxin.
  – Endotoxin contributes to disease symptoms such as fever and shock.

Other stain types

• Gram-staining is not appropriate for all bacterial strains.
• We show an image of mycobacteria, which does not have a cell wall and is visualized with acid-fast staining methods.
• We show Treponema pallidum, which is a bacterial strain that has very thin cell wall but can be seen with dark-field microscopy, as in our image, or fluorescent antibodies.
• Some strains, such as the intracellular Chlamydiae, can be seen with hematoxylin and eosin (H&E) staining.

MORPHOLOGY

• The bacterial cell wall contributes to its morphology, which is also used for classification purposes.

Cocci – Spherical

• Clusters
  – Ex: Staphylococcus species
• Chains
  – Ex: Streptococcus pyogenes
• Pairs
  – Ex: Pointed, like Streptococcus pneumoniae
  – Ex: Coffee-bean shaped, like Neisseria gonorrhoeae.

Bacilli – Rods

• Rectangular ends
  – Ex: Bacillus anthracis
• Rounded ends
  – Ex: Salmonella
• Club-shaped
  – Ex: Corynebacterium diptheriae (“Coryne” means “club”)
• Fusiform-shaped
  – Ex: Fusobacterium nucleatum (associated with periodontal disease)
• Bent/comma-shaped
  – Ex: Desulfovibrio desulfuricans (associated with the gut)

Coccobacilli – Intermediate

• Short, rounded rods
  – Ex: Coxiella burnetii (Q fever)

Spirochetes – Spirals

• Ex: Treponema pallidum (Syphilis)

Be aware that there is intertextual variation in regards to bacterial classification schema, and some authors describe more or fewer morphological types.