Bacteria

Overview

• Bacteria live almost everywhere.

• Most are microscopic ranging from 0.5 – 5 m in size, and unicellular.

• They have a variety of shapes when viewed under a microscope, most commonly:

– Spheres, Rods, Spirals

• There are commonly found arrangements of bacteria based on their division.

Typical bacterial cell

Fimbriae

Bacterial DNA

(Single chromosome)

Ribosomes

Plasma membrane

Cell wall

Capsule

Flagella

Fig. 27-2

(a) Spherical(cocci)

1 µm

(b) Rod-shaped(bacilli)

2 µm

(c) Spiral

5 µm

Sphere

• Single coccus, plural cocci

• Diplococci – in pairs

• Streptococci – in chains

• Staphylococci – in irregular clusters

Rod

• Single bacillus, plural bacilli

• Diplobacilli – in pairs

• Streptobacilli – in chains

Spiral

• These bacteria range in size from 1m to over 100 m

• Bacteria maintain their cell shape, and are protected from their physical environment by their cell wall.

• Bacterial cell walls contain a material called peptidoglycan.

• Using the Gram stain, scientists classify many bacteria into Gram-positive and Gram-negative groups based on cell wall composition

• Gram-negative bacteria have less peptidoglycan and an outer membrane that can be toxic.

• Many antibiotics target peptidoglycan and damage bacterial cell walls, so Gram-negative bacteria are more likely to be antibiotic resistant.

Fig. 27-3

Cellwall

Peptidoglycanlayer

Plasma membrane

Protein

Gram-positivebacteria

(a) Gram-positive: peptidoglycan trapscrystal violet.

Gram-negativebacteria

(b) Gram-negative: crystal violet is easily rinsed away,revealing red dye.

20 µm

Cellwall

Plasma membrane

Protein

Carbohydrate portionof lipopolysaccharide

Outermembrane

Peptidoglycanlayer

Fig. 27-3a

Cellwall

Peptidoglycanlayer

Plasma membrane

Protein

(a) Gram-positive: peptidoglycan trapscrystal violet.

Fig. 27-3b

Cellwall Peptidoglycan

layer

Plasma membrane

Protein

(b) Gram-negative: crystal violet is easily rinsedaway, revealing red dye.

Outermembrane

Carbohydrate portionof lipopolysaccharide

Fig. 27-3c

Gram-positivebacteria

Gram-negativebacteria

20 µm

• A polysaccharide or protein layer called a capsule covers many bacteria.

Fig. 27-5

Fimbriae

200 nm

Some bacteria have fimbriae or attachment pili which allow them to stick to surfaces or other bacteria in a colony.

• Bacteria are motile, meaning they can move around.

• Most bacteria use flagella to propel themselves.

• Bacteria are prokaryotes, pro- meaning before and kary- meaning nucleus. They do not have compartments or membrane bound organelles.

• Their genome is made of a single circular chromosome.

• Some species of bacteria also have smaller rings of DNA called plasmids.

• Bacteria reproduce quickly by binary fission and can divide every 1-3 hours.

• Many bacteria can also form inactive endospores, which means they can remain viable in harsh conditions for centuries.

• Endospores form when the environment is too harsh.

Fig. 27-9

Endospore

0.3 µm

• Even though bacteria reproduce asexually, they have an immense amount of genetic diversity.

• There are three factors that contribute to this genetic diversity:

– Rapid reproduction

– Mutation

– Genetic recombination

• In a process called transformation, a bacterial cell can take in and incorporate foreign DNA from the surrounding environment.

• Conjugation is the process where genetic material is transferred between bacterial cells.

• Sex pili allow cells to connect to transfer DNA

Fig. 27-12

Sex pilus 1 µm

Plasmid BacteriumPilus

ChromosomeDonor

Recipient

Plasmid is replicated

New donor New donor

• Many bacteria have plasmids that carry genes for antibiotic resistance.

• Antibiotics kill bacteria that aren’t resistant, but the bacteria that are resistant survive, and have less competition.

• Antibiotic resistant strains of bacteria are becoming more common.

How bacteria cause disease

• Pathogenic bacteria typically cause disease by releasing exotoxins or endotoxins

• Exotoxins cause disease even if the bacteria that produce them are not present.

– Example – botulinum toxin produced by Clostridium botulinum

– Botox

• Exotoxins fall into three categories:

– Cytotoxins – kill cells

– Neurotoxins – interfere with normal nerve impulse -such as botulism Clostridium botulinum – results in muscle paralysis - botox

– Enterotoxins – affect cells lining the gastrointestinal tract

• Exotoxins can be inactivated by heat or chemicals and can no longer cause disease, but they can be used to create toxoids.

• Toxoids are injected to stimulate the production of antitoxins and provide immunity – vaccine.

• Endotoxins are released only when Gram-negative bacteria die and their cell walls break down

– Example – Escherichia coli

• Toxins are the primary factor in pathogenicityor ability to cause disease.

• There are 220 known bacterial toxins and almost half cause damage to the cell membrane.

• Bacteria have caused some of the most widespread epidemics in human civilization.

Bubonic Plague

• Bubonic plague is caused by the bacteria Yersinia pestis. It is a Gram-negative rod-shaped coccobacillus.

• It is transmitted to humans from animals primarily by rats that are infected by fleas.

• The plague or bubonic form of infection is very severe with a fatality rate of 30 – 60 % if left untreated.

• The name bubonic comes from the appearance of a “bubo” which is a swollen lymph node that can become an open sore as the infection advances.

• The bubonic plague that occurred in 14th

century Europe is one of the most destructive pandemics in history resulting in between 100 and 200 million deaths.

• Antibiotics or antibacterials are drugs used to treat infections caused by bacteria.

• Antibiotics work in one of two ways:

– Bacteriocidal – antibiotic kills bacteria

– Bacteriostatic – stops bacteria from multiplying

• Many antibiotics are naturally occuringchemicals made by other microorganisms.

• The first antibiotic was discovered in 1928 by Sir Alexander Fleming.

• He made a chance discovery while looking at agar plates he was going to throw away. He discovered a plate growing Staphylococcus aureus that had been contaminated. The mold contamination had killed the S. aureus.

• Penicillin works by targeting the bacterial cell wall, an causing it to rupture. Many antibiotics target key bacterial functions or ability to grow and divide.

• Antibiotic resistance occurs when populations of bacteria stop being sensitive to an antibiotic.

• The evolution of resistant strains is a natural phenomenon that happens when microorganisms are exposed to antibiotics and resistant traits can be exchanged.

•However the misuse of antibiotics speeds up this natural process.