Betsy T., Keogh J. Microbiology Demystified

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parts of the body (for example, through the ingestion of water that is

contaminated with fecal matter.)

• Shigella. Shigella causes bacillary dysentery or shigellosis, more com-

monly known as traveler’s diarrhea.

• Salmonella. Salmonella is a group (genera) of enteric bacteria that has

members that can infect humans. Species include Salmonella typhi,

which causes typhoid fever and Salmonella choleraesuis and

Salmonella enteritides both of which are food-borne pathogens that

cause salmonellosis, a type of food poisoning.

• Klebsiella. The genera Klebsiella is a bacterium that cause bacterial

pneumonia.

•Erwinia. Erwinia is a bacterium that infects plants and causes what is

commonly called soft root rot.

• Enterobacter. The genus enterobacter consists of the species Enterobacter

cloace and Enterobacter aerogenes. Both of these organisms cause uri-

nary tract infections and nosocomial (hospital) infections in individuals

with a weakened immune system.

• Serratia marcescens, Serratia marcescens bacteria are found on catheters

and instruments that are allegedly sterile; this bacterium causes urinary

and respiratory infections.

•Yersinia pestis. Yersinia pestis, also known as Y.pestis, caused the

bubonic or black plague, which ravaged Europe during the Middle

Ages. It begins by causing abscesses of lymph nodes and then produces

pneumonia-like symptoms when it reaches the lungs, which is called

pneumonia plague.

•

Vibrios. Vibrios are facultative anaerobic gram-negative, comma-shaped

bacteria that inhibit aquatic environments and some also live in the intes-

tinal tracts of animals and humans. There are two important species of vib-

rios bacteria. These are:

•Vibrio cholerae. Vibrio cholerae is the bacteria that cause cholera, signs

and symptoms are abdominal pain and watery diarrhea.

•Vibrio parahaemolyticus. Vibrio parahaemolyticus is the bacterium

that causes inflammation and irritation of the stomach and intestine—

better known as gastroenteritis—when contaminated shellfish is ingested

raw or undercooked.

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•

Pasteurella-Haemophilus. Pasteurella-Haemophilus are very small faculta-

tively anaerobic gram-negative, rod shaped bacteria, that are named for

Louis Pasteur. Examples include:

• Pasteurella causes blood poisoning (septicemia) in cattle, chickens

(fowl cholera) and pneumonia in various animals.

• P. multocida, the species which was identified by Louis Pasteur, is

transmitted to humans by a dog or cat bite.

• Haemophilus. The genus Haemophilus (which means “blood loving”)

are bacteria that live in mucous membranes of the upper respiratory

tract, mouth, intestinal tract and vagina.

• H. ducreyi, the species of Haemophilus bacterium that causes chan-

croid, which is an infectious venereal ulcer.

• H. aegyptiusis the species that causes acute conjunctivitis or pink eye.

Anaerobic Gram-Negative Cocci and Rods

Anaerobic gram-negative cocci and rods can live in anaerobic conditions and are

non-motile. They do not form endospores, which are small spores that developed

inside the bacteria as a resistant survival form of the bacteria.

Anaerobic gram-negative cocci are spherically shaped and form a single

chain or are clustered. Veillonella are common anaerobic gram-negative cocci

that are found between teeth and on gums. Veillonella is the cause of abscesses

of teeth and gums.

Anaerobic gram-negative rods are called Bacteroides and are members of the

Bacteriodaceae family of bacteria that live in the intestinal tract of humans.

Bacteroides can cause peritonitis, which is inflammation of the peritoneum due

to infection. Another kind of anaerobic gram-negative rod is Fusobacterium.

These are long slender rods that live in the gingival crevices of teeth and cause

gingivitis, which is a gum infection.

Rickettsias and Chlamydias

Rickettsias and Chlamydias are intracellular parasites that need a host in order

to reproduce and therefore enter the cell of a host. These bacteria were once

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thought to be viruses that invaded cells. They are classified as bacteria because

they have bacterial cell walls and contain DNA and RNA, which is not the case

with a virus. Rickettsias and Chlamydias have no means of mobility because

they lack flagella. They are also gram-negative.

Rickettsias are small rod-shaped or spherical bacteria that live in the cells of

ticks, lice, fleas, and mites (arthropods) and can be transmitted to humans when

bitten by arthropods, causing rickettsial disease. Rickettsial disease causes cap-

illaries to become permeable resulting in a rash. Rickettsias reproduce by binary

fission where a cell wall forms across the cell and the two halves separate to

become individual cells.

Common Rickettsias are:

•

Rickettsia prowazekii. Rickettsia prowazekii is transmitted by lice and

causes endemic typhus.

•

R. rickettsii. R. rickettsii is transmitted by ticks and causes Rocky Mountain

spotted fever.

•

R. tsutsugamushi. R. tsutsugamushi is transmitted by arthropods and cause

scrub typhus that presents with fewer, rash and inflammation of the lymph

nodes.

•

Coxiella burnetti. Coxiella burnetti is transmitted by aerosols or contami-

nated milk and causes Q fever, which is similar to pneumonia.

•

Bartonella bacilliformis. Bartonella bacilliformis is transmitted by arthro-

pods and causes a wart-like rash called Oroya fever.

Chlamydias are very small spherical or coccoid bacteria that are non-motile

and can be transmitted from person-to-person contact or by airborne respiration.

Chlamydias are not transmitted by arthropods. They are so small that they mul-

tiply in host cells. There are three species of chlamydia:

•

Chlamydia trachomatis. C.trachomatis causes trachoma, which is a com-

mon cause of blindness and the non-gonorrhea sexually transmitted dis-

ease urethritis (inflammation of the urethra).

•

C. penemoniae causes a mild form of pneumonia in adolescence.

•

C. psittaci causes psittacosis.

Mycoplasmas

Mycoplasmas are very small facultatively anaerobic bacteria (some are oblig-

ately anaerobic) that have taken on many shapes (pleomorphic) and were once

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thought to be viruses because they lack a cell wall. However, they have a cell

membrane, DNA and RNA, which distinguishes them from viruses.

Mycoplasmas can also resemble fungi because some Mycoplasmas produce

filaments that are commonly seen in fungi. It is these filaments that led scien-

tists to name it Mycoplasma. Myco means “fungus.”

Many Mycoplasmas are unable to move by themselves because they do not

have flagella, but some are able to glide on a wet surface.

Two of the more common types of Mycoplasma are:

•

Mycoplasma pneumoniae. Mycoplasma pneumoniae is the cause of atypi-

cal pneumonia, commonly referred to as walking pneumonia.

•

Ureaplasma urealyticum. Ureaplasma urealyticum is a bacterium that is

found in urine and one that can cause urinary tract infection.

Gram-Positive Cocci

Within the gram-positive cocci section are two genera. These are Staphylococcus

and Streptococcus, and each has an important role in medicine.

•

Staphylococcus. Staphylococcus bacteria have a grape-like cluster appear-

ance and grow in environments of high osmotic pressure and low moisture.

Osmotic pressure is the pressure required to prevent the net flow of water

by osmosis. Infections caused by the Staphylococcus bacteria are typically

called staph infections. Here are the commonly found Staphylococcus

bacteria:

• Staphylococcus aureus. Staphylococcus aureus, also called S. aureus, is a

bacterium that forms yellow-pigmented colonies that grows with oxygen

(aerobically) or without oxygen (anaerobically). S. aureus is the cause of

toxic shock syndrome that results in high fever, vomiting and sometimes

death. It produces enterotoxins, which affects intestinal mucosa. S. aureus

is also the cause of boils (skin abscess), impetigo (pus-filled blisters on

the skin), styes (an infection at the base of an eye lash), pneumonia,

osteomyelitis, acute bilateral endocarditis (inflammation of the internal

membranes of the heart) and scalded skin syndrome in very young chil-

dren that causes skin to strip off (denude) due to an exfoliative toxin.

S. aureus is one of the major types of infections that occur in hospi-

tals because it is resistant to antibiotics such as penicillin. An infection

of S. aureus is usually identified by the presence of an abscess.

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• Staphylococcus epidermis: Staphylococcus epidermis, also known as S.

epidermis, is the frequent cause of urinary tract infections in the elderly

and also causes subacute bacterial endocarditis, which is a chronic in-

fection of the endocardium (a thin layer of connective tissue that lines

the chambers of the heart) and heart valves.

• Staphylococcus saprophyticus. Staphylococcus saprophyticus, also

known as S. saprophyticus, causes urinary tract infections, usually in

adolescent girls.

•

Streptococcus. Streptococcus bacteria appear as a single, paired or chained

spherical gram-positive bacteria. Streptococci do not use oxygen, though

most are aerotolerant. Few may be obligately anarobic. Infections caused

by the Streptococcus bacteria are generally referred to as a strep infection.

Microbiologists classify Streptococcus bacteria in three ways.

• The type of hemolysis (destruction) of red blood cells caused by the

Streptococcus bacteria. There are three types characterized by:

• Alpha-hemolytic group. Incomplete lysis (destruction of the cell)

within green pigment surrounding the colony.

• Beta-hemolytic group. Total lysis and a clear area around the colony.

• Gamma-hemolytic group. Absence of lysis. This group is of no clin-

cal importance.

• The Lance Field classification. There are four groups:

• Group A Streptococci. Characterized by Streptococcus pyogenes and

secrete of erthrogenic exotoxins responsible for scarlet fever.

• Group B Streptococci. Characterized by Streptococcus agalactiae,

which is part of normal oral and vaginal flora and causes urogenital

(urinary and reproductive systems) infections in females.

• Group C Streptococci. Causes animal diseases.

• Group D Streptococci. Characterized by Streptococcus faecalis, which

is a normal part of oral and intestinal flora. Diseases of S. faecalis are

endocarditis, urinary tract infections and septicemia (blood poisoning).

• Ungrouped Streptococci. There are two kinds:

• Viridans streptococci. Characterized by Streptococcus viridans and

Streptococcus salvarius, which causes subacute bacterial endocardi-

tis, and Streptococcus mutans which causes a biofilm called plague

resulting in tooth decay.

• Pneumococcal streptococci. Characterized by Streptococcus pneu-

moniae which causes lobar pneumonia and otitis media (middle ear

infection).

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Endospore-Forming Gram-Positive Rods and Cocci

Endospore forming, gram-positive rods and cocci consist mainly of rod-shaped

bacteria of the genera Bacillus and Clostridium. Another cocci bacteria included

in this group are of the genus Sporosarcina. These bacteria have no clinical sig-

nificance and are saprophytic soil bacteria. Saphrophytes are organisms that feed

on dead organic matter.

These bacteria can be strict aerobes, facultative anaerobes, obligate anaerobes

or microaerophiles. Microaerophiles are bacteria that grow best in an environ-

ment that has a small amount of free oxygen.

The formation of endospores by bacteria is important in medicine and the food

industry because these endospores are resistant to heat and many chemicals.

There are three genera in this section. These are:

•

Bacillus: Bacillus consists of the following bacterium:

• Bacillus anthracis. Bacillus anthracis causes anthrax, a severe blood

infection that infects cattle, sheep and horses and can be transmitted to

humans. B. anthracis is a non-motile facultative anaerobe and produces

exotoxin. Anthrax can result in central nervous system distress, respira-

tory failure, anoxia and death.

• Bacillus cereus. Bacillus cereus produces enterotoxin (a toxin that affects

the intestine) and causes gastroenteritis (food poisoning).

• Bacillus thuringiensis. Bacillus thuringiensis produces a toxin that attacks

the digestive system of insects, causing the insects to stop feeding by

causing paralysis of the insects guts.

•

Sporosarcina. Sporosarcina are bacteria that inhabit the soil and receive

nutrients by feeding on dead organic matter.

•

Clostridium. Clostridium are rod-shaped bacteria that exist in water, soil,

and in the intestinal tract of animals and humans. These bacteria do not

require oxygen. They release toxins that cause disease. Here are the com-

mon types of Clostridium:

• Clostridium tetani. Clostridium tetani, also known as C. tetani, causes

tetanus, commonly referred to as lockjaw.

• Clostridium difficile. Clostridium difficile, also known as C. difficile,

causes gastroenteritis.

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• Clostridium perfringes. Clostridium perfringes, also known as C.

perfringes, causes myonecrosis—better known as gas gangrene—which

produces hydrogen gas in deep tissue wounds, resulting in cell death.

• Clostridium botulinum. Clostridium botulinum, also known as C. botu-

linum is a cause of food poisoning usually as a result of improperly

canned food. It produces an exotoxin that causes flaccid paralysis

(weakness of muscle tone) due to the suppression of acetylcholine,

which is a neurotransmitter. The result is vomiting, difficulty speaking,

and difficulty swallowing, which can lead to respiratory paralysis and

death. Physicians use Clostridium botulinum as a neural block that

inhibits muscle contraction. Clostridium botilinum is also used cosmet-

ically to relax muscles that cause facial wrinkles (Botox injections). The

C. botulinum toxin blocks the exocytosis of synaptic vesicle of the neu-

romuscular junction, where motor neurons meet muscle.

Regular Nonsporing Gram-Positive Rods

Regular, non-sporing gram-positive rods are obligate or facultative anaerobes

that have a rod-shaped appearance as is implied by its name. They inhabit fer-

menting plants and animal products. There are four genera within this section.

These are:

•

The genus, Lactobacillus are non-sporing gram-positive rods. These are

aerotolerant bacteria that produce lactic acid from simple carbohydrates.

The acidity inhibits competing bacteria. An example of a Lactobacillus

organism is the species Lactobacillus acidophilus.

•

Lactobacillus acidophilus is found in the human intestinal tract, oral cav-

ity and adult vagina. They produce an acidic environment that inhibits the

growth of harmful bacteria by fermenting glycogen into lactic acid.

Lactobacillus acidophilus is also used commercially to produce an assort-

ment of food products including sauerkraut, pickles, buttermilk and yogurt.

Other examples of regular non-sporing gram-positive rod bacteria are:

•

Listeria monocytogenes. Listeria monocytogenes contaminates food and

dairy products, if ingested can cause the disease listeriosis, which causes

an inflammation of the brain and meninges (meningitis).

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•

Erysipelothrix rhusiopathiae. Erysipelothrix rhusiopathiae causes erysi-

peloid, which is red, swollen and painful lesions, frequently seen in fisher-

men and meat handlers.

Irregular Nonsporing Gram-Positive Rods

These bacteria are irregular, non-sporing rods. Although this group of bacteria

are generally rod-shaped, their shape can vary (pleomorphic). Some resemble a

club while others are long, threadlike cylinders. There are three genera within

this section. These are:

•

Corynebacteria. Corynebacteria are club-shaped and receive nutrients from

dead or decaying organic material (saprophytes). Corynebacteria inhabit

airy soil and water and cause diphtheria. Corynebacteria diphtheriae is the

organism which causes diphtheria.

•

Propionibacterium. Propionibacterium infects wounds and causes abscesses.

An example would be Propionibacterium acnes.

•

Actinomycetales. Actinomycetales is a long, threadlike cylinder (filament)

that inhibits soil and some provide nitrogen to plants. The species

Actinomycetale israelii, which causes actinomycosis, which destroys tis-

sues in the jaw, head, neck, and lungs. Actinomycetales was originally

classified as a fungus because of its shape.

Mycobacteria

Mycobacteria require oxygen (aerobic) and are acid-fast organisms that remain

red while most are blue. Large amounts of lipids in the Mycobacteria’s cell enve-

lope, resists basic dyes. Myco, which means “fungus-like” is how this organism

got its name.

•

Mycobacterium tuberculosis. Mycobacterium tuberculosis causes tuberculosis.

•

Mycobacterium leprae. Mycobacterium leprae, also known as M. leprae,

causes Hansen’s disease (leprosy).

•

Mycobacterium bovis. Mycobacterium bovis, also known as M. bovis,

causes tuberculosis in cattle and can be transmitted to humans.

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Nocardia Forms

Nocardia are a group of a long thread-like cylinder shaped bacteria that inhabits

soil and need oxygen to grow (aerobic). They are gram-positive and cannot

move by themselves (they are non-motile).

Nocardia asteroids, also known as N. asteroids, is a common bacterium

within this group. It causes mycetoma, which causes abscesses on the hands and

feet and also causes lung infection.

Quiz

1. Archaea can exist in very hot and very cold environments, making them

a resilient microorganism that can survive attacks and destroy other

bacteria.

(a) True

(b) False

2. Archaea that function in the presence of air are called

(a) methanogenic archaea

(b) cell wall–less archaea

(d) extreme halophiles

3. Archaea that live in an extreme salty environment are called

(a) methanogenic archaea

(b) cell wall–less archaea

(d) extreme halophiles

4. Single-celled archaea that produce methane and carbon dioxide (CO

) by

fermenting simple organic carbon compounds or oxidizing H

without

oxygen to produce CO

are called

(a) methanogenic archaea

(b) cell wall–less archaea

(d) extreme halophiles

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5. Archaea that do not have cell walls are called

(a) methanogenic archaea

(b) cell wall–less Archaea

(d) extreme halophiles

6. “Aerobic/microaerophilic” is a designation for bacteria that require small

amounts of oxygen to grow.

(a) True

(b) False

7. Rod-shaped bacteria with flagella at both ends of the rod, which give the

bacteria mobility, are called

(a) Pseudomonas aeruginosa

(b) Legionella pneumophilia

(d) Legionella micdadei

8. The bacteria that infects lungs and causes a strain of pneumonia com-

monly called Pittsburgh pneumonia is known as

(a) Pseudomonas aeruginosa

(b) Legionella pneumophilia

(d) Legionella micdadei

9. A pathogenic bacterium that infects the urinary tract and wounds in

humans is called

(a) Pseudomonas aeruginosa

(b) Legionella pneumophilia

(d) Legionella micdadei

10. This bacterium, discovered in 1976, infected and killed members of the

American Legion at their convention in Philadelphia.

(a) Pseudomonas aeruginosa

(b) Legionella pneumophilia

(d) Legionella micdadei

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