The World of Bacteria
What does a bacterium look like?
Internal Structures:
cytoplasm
nucleoid
ribosomes
Boundaries:
cell membrane
cell wall
capsule
Appendages:
flagellum
pili
Shapes of Bacteria
Bacteria Identification Criteria
Shapes
Growth Patterns
Bacilli
“Prefixes describe pattern”
Cocci
Sprilli
“strepto…. Means in chains
“staphylo…Means in clusters
Examples:
Streptococcus pneumoniae
Bacterial Growth Patterns:
Streptococcus
Bacterial Growth Patterns:
Staphylococcus
The Gram Stain
Christian Gram (1884) develops gram
staining process.
Use of two stains: crystal violet and
safranine
Stains interact differently with a particular
type of cell wall.
Cells will be either Gram positive or Gram
negative.
How else are bacteria identified? Another
criterion? Composition and Construction
of the Cell Wall
Gram Negative
The Gram-negative cell wall is composed of a thin, inner layer of peptidoglycan and an outer
membrane consisting of molecules of phospholipids, lipopolysaccharides (LPS), lipoproteins and
sutface proteins. The lipopolysaccharide consists of lipid A and O polysaccharide.
Gram Staining Process
What color are gram positive?
What color are gram negative?
Which is Gram positive?
Which is Gram negative?
Gram negative: A group of bacteria that do not retain the crystal violet dye
after the differential staining procedure known as Gram staining. They appear
pink due to the counterstain, safranin. Gram positive appears purple. The
difference between Gram negative and Gram positive bacteria is the cell wall
structure, which accounts for the different staining characteristics
The Gram Stain Results:
Gram Positive
Gram Negative
So what is the medical significance of this technique?
What are antibiotics?
Antibiotics are strong medicines.
Antibiotics only work against infections caused by bacteria.
Antibiotics kill bacteria or stop them from growing.
Antibiotics should be used wisely.
Some Clinical Antibiotics
Antibiotic
Producer organism
Activity
Site or mode of
action
Penicillin
Penicillium chrysogenum
Gram-positive bacteria
Wall synthesis
Cephalosporin
Cephalosporium acremonium
Broad spectrum
Wall synthesis
Bacitracin
Bacillus subtilis
Gram-positive bacteria
Wall synthesis
Polymyxin B
Bacillus polymyxa
Gram-negative bacteria
Cell membrane
Erythromycin
Streptomyces erythreus
Gram-positive bacteria
Protein synthesis
Neomycin
Streptomyces fradiae
Broad spectrum
Protein synthesis
Streptomycin
Streptomyces griseus
Gram-negative bacteria
Protein synthesis
Tetracycline
Streptomyces rimosus
Broad spectrum
Protein synthesis
Vancomycin
Streptomyces orientalis
Gram-positive bacteria
Protein synthesis
Gentamicin
Micromonospora purpurea
Broad spectrum
Protein synthesis
Rifamycin
Streptomyces mediterranei
Tuberculosis
Protein synthesis
Antibiotic
Sensitivity
Figure F. Antibiotic-sensitivity testing. Petri dishes were spread-inoculated with Staphylococcus
albus (white growth) or Micrococcus luteus (yellow growth) before antibiotic assay "rings" were
placed on the agar surface. The coloured disks at the end of each spoke of the rungs are
impregnated with different antibiotics. Clockwise from the top (arrow) these are: Novobiocin,
Penicillin G, Streptomycin (white disk), Tetracycline, Chloramphenicol, Erythromycin, Fusidic
acid (green disk) and Methicillin. Clear zones of suppression of bacterial growth around the
individual antibiotic disks are evidence of sensitivity to these antibiotics.
The diameter of the clear zone is related to the initial antibiotic concentration (which differs for
the antibiotics on the ring), its solubility and its diffusion rate through agar. Standard tests
performed on many bacteria by the manufacturers of these assay disks enable the diameter of
the clearing zone to be related to the minimum inhibitory concentration (MIC) of each antibiotic
for the strain being tested. The MIC can then be compared with the known tissue levels of these
antibiotics when they are administered to patients, to assess whether the antibiotics would be
effective for treatment of particular pathogens.
Antibiotic Resistant Bacteria
Bacteria that is not affected by an antibiotic.
Antibiotic resistance is a phenotype. They posses a gene
that renders them resistant (genotype).
Antibiotic resistant gene produces enzymes that
breakdown the antibiotics.
How Antibiotic Resistant Bacteria Develop
http://www.fda.gov/fdac/features/795_antibio.html
Antibiotic resistance results from gene action. Bacteria acquire genes
conferring resistance in any of three ways.
In spontaneous DNA mutation, bacterial DNA (genetic
material) may mutate (change) spontaneously (indicated by starburst). Drugresistant tuberculosis arises this way.
In a form of microbial sex called transformation,
one bacterium may take up DNA from another bacterium. Penicillin-resistant
gonorrhea results from transformation
One More Way towards Resistance
Most frightening, however, is resistance
acquired from a small circle of DNA called a plasmid, that can flit from one
type of bacterium to another. A single plasmid can provide a slew of different
resistances. In 1968, 12,500 people in Guatemala died in an epidemic of
Shigella diarrhea. The microbe harbored a plasmid carrying resistances to
four antibiotics!
What on the plasmid makes the bacteria resistant?
Resistant Genes
Genes are located in a circular piece of DNA found
in the bacterial cell called a plasmid.
Bacterial also has chromosomal DNA.
Binary Fission: Asexual Reproduction