Fundamentals of Microbiology

Question 1

Molds:

Answer 1

• may have more than one nucleus in a cell.
• can be pathogens of plants, animals or humans.
• may produce macroscopic fruiting structures.
• may produce air-borne spores.

Molds may have cells that contain several nuclei (aseptate forms) or that have nuclei separated from each other (septate forms). Molds frequently use the air-borne route for dispersal of their spores. They may be pathogenic for plants as well as for animals and humans. Mushrooms, toadstools, bracket fungi and puffballs are examples of macroscopic fungal fruiting structures. Peptidoglycan is a uniquely bacterial polymer.

Question 2

Viruses are classified by:

Answer 2

• morphology.
• genomic nucleic acid.

Viruses are classified on their morphology, genomic material and the organization of their genome. Their hosts and diseases caused are not now considered relevant. A good example is the disease hepatitis where there are at least five completely different viruses that cause hepatitis in man. Each virus belongs to a different family.

Question 3

The cell walls of Gram-positive bacteria:

Answer 3

• have a thick layer of peptidoglycan.
• contain teichoic acid.

Gram-positive bacteria have up to forty layers of peptidoglycan in their cell walls: Gram-negative bacteria have only one or two. In Gram-negative cells, however, beyond the peptidoglycan is the periplasm and beyond that is an outer membrane. These structures are absent from Gram-positive bacteria. Teichoic acids are major antigenic components of the cell walls of Gram-positive bacteria.

Question 4

The bacterial cytoplasmic membrane:

Answer 4

• may contain respiratory enzymes and ATPase.
• contains specific protein permeases.
• contains phospholipids which are organized in a bilayer.
• is a flexible structure at ambient temperature.

It is a general property of biological membranes that they are flexible at the ambient temperatures at which the organism, from which they are derived, grows and we believe biological membranes adopt a phospholipid bilayer structure at such temperatures. In order to permit entry and exit of nutrients, etc. biological membranes have special protein channels to act as permeases. The electron transport chain and ATPase used in respiration are located within the cytoplasmic membrane of bacteria capable of respiratory metabolism. In eukaryotic organisms, these are located on the mitochondrial inner membrane.

Question 5

Among the bacteria having cell walls from which peptidoglycan is absent are:

Answer 5

• mycoplasmas.
• chlamydia.

Although spirochaetes are flexible, they have peptidoglycan cell walls of Gram-negative structure. Staphylococci and clostridia are both Gram-positive and have cell envelopes that include many layers of peptidoglycan. Chlamydia and mycoplasmas use other means to maintain cell integrity or rigidity.

Question 6

Genera of bacteria that form endospores include:

Answer 6

• Bacillus.
• Clostridium.

Endospores are formed by bacteria of the genus Bacillus and the genus Clostridium. Lactobacilli are non-sporing Gram-positive bacilli that may grow aerobically and anaerobically. Staphylococci are facultative non-sporing Gram-positive cocci and members of the genus Bacteroides are obligately anaerobic Gram-negative bacilli that do not form spores.

Question 7

Animal viruses:

Answer 7

• have proteins that are responsible for attachment to the cell surface.
• may be taken up by a cell by endocytosis.
• may fuse directly with a cell membrane.

All animal viruses exhibit cell tropism in vivo, which means that structural elements of the virus particle direct the infectious agent to specific cell types. Animal viruses utilize normal cell processes for protein and nucleic acid uptake.

Question 8

The following produce spores that can be readily observed microscopically:

Answer 8

• Bacillus anthracis.
• Clostridium botulinum.
• Clostridium tetani.

Spore production is a characteristic of the genus Bacillus and of the genus Clostridium. The genus Bacillus comprises aerobic spore-formers, although a few members of this genus grow in anaerobic conditions. The genus Clostridium comprises anaerobic sporing bacteria. All members of these genera form spores, but Clostridium perfringens does not spore readily when grown in artificial culture.

Question 9

The following bacteria appear as cocci when viewed microscopically:

Answer 9

• Neisseria gonorrhoeae.

Only cells of Neisseria gonorrhoeae appear as cocci when viewed microscopically. Both Escherichia coli and Clostridium tetani are rod-shaped (bacilli). Campylobacter jejuni appears as a vibrio (curved rod), and Treponema pallidum is a spirochaete.

Question 10

Protists:

Answer 10

• maybe free-living.
• are all unicellular.
• may produce cilia.
• may produce flagella.
• ingest their food.
• do not have rigid cell walls.

Protists, by definition, are unicellular and may reproduce asexually. Some are ciliate and others are flagellate. Many are free-living. Protists produce resting cells known as cysts. While many do not cause disease, some cause life-threatening infections and are thus highly pathogenic.

Question 11

A microorganism will grow in the temperature range -2 to 22°C under a normal atmosphere - the microorganism is:

Answer 11

• a pyschrophile.

Microorganisms capable of growing at only low temperature, approximately 25°C and below are defined as pyschrophiles. The mesophilic temperature range is approximately between 10 and 45°C and a thermophile will grow above approximately 45°C.

Question 12

Peptidoglycan is:

Answer 12

• found in both Gram-positive and Gram-negative cell walls.
• responsible for maintaining the shape of many bacteria.
• a component of the bacterial cell wall.

Peptidoglycan is only found in the cell walls of bacteria and never in fungi or other eukaryotic cells. It is a rigid and very strong polymer that lends the bacterial cell its shape.

Question 13

The outer membrane of Gram-negative bacteria contains:

Answer 13

• proteins.
• lipopolysaccharides.
• phospholipids

The outer membrane, characteristic of Gram-negative cells, lies beyond the periplasm and this lies above the peptidoglycan layer. As is typical of biological membranes, it contains phospholipids and proteins, but its outer layer also carries lipopolysaccharide, the major somatic antigen of Gram-negative bacteria.

Question 14

In categorizing bacteria as Gram-negative or positive:

Answer 14

• the cell envelope composition determines the reaction.
• the reaction is independent of cell shape.
• there is an association between the stain reaction and the susceptibility to certain antibiotics.

Gram-positive cell walls contain more peptidoglycan than Gram-negative bacteria, and the Gram-negative cell is surrounded by an additional outer membrane. Bacteria exhibiting either Gram reaction can be round (cocci) or rod-shaped (bacilli). It is not the cell wall that is stained with the Gram-stain, but its structure determines whether or not the cell can retain the crystal violet-iodine complex. Because of the fundamental differences in the cell structure, there is an association with the Gram reaction and susceptibility to certain antibiotics. The Gram protocol requires heat fixed bacteria and exposure to organic solvents. This treatment kills all bacteria, some of which appear Gram-negative upon subsequent Gram staining.

Question 15

Virus particles:

Answer 15

• need not to contain RNA.
• contain only one kind of nucleic acid.
• may contain lipid envelopes.

Virus particles can infect eukaryotic cells, but are not, themselves eukaryotes, since they do not contain a membrane-bound nucleus. Indeed, their simple structure means that they are akaryotic. They contain only DNA or RNA, although during the replication cycle of retroviruses, a DNA copy of the RNA genome is generated, and becomes integrated into the host DNA. Some viruses have a lipid-containing envelope.

Question 16

The hyphae of fungi:

Answer 16

• form an interwoven mass: the mycelium.
• may aggregate to form tissues.
• bear asexual spores on sporophores.
• are mostly saprophytes (living on dead matter).
• can be divided into yeasts and molds.
• are eukaryotes.

Hyphae may be uninucleate, binucleate or multinucleate cells depending on the fungus and stage of growth. Hyphae extend at the tip. The cells of fungi have membrane-bound nuclei and organelles and so fungi are eukaryotic. Yeasts are unicellular fungi that multiply by a process of “budding”: molds are filamentous fungi. Many fungi are microscopic, but molds can grow to be very large, as with bracket fungi, mushrooms, toadstools, and puffballs. Fungi have cell walls of a different structure from the cell walls of either bacteria or plants. Many fungi live on dead organic matter and are thus saprophytes.

Question 17

Peptidoglycan contains:

Answer 17

• amino acids.
• N-acetylglucosamine.
• N-acetylmuramic acid.

Peptidoglycan is a polymer component of the bacterial cell wall and comprises a backbone of alternating N-acetylmuramic acid and N-acetylglucosamine which is cross-linked by peptide chains composed of amino acids. It is a unique biopolymer, containing both D- and L-amino acids.

Question 18

Yeasts:

Answer 18

• may have polysaccharide capsules.
• may reproduce by budding.
• are predominantly unicellular.

Yeast is unicellular fungi that typically reproduce by budding. They do not produce air-borne spores. Cryptococcus neoformans produces a polysaccharide capsule that protects it when it causes meningitis. Peptidoglycan is a uniquely bacterial polymer.

Question 19

Fungi:

Answer 19

• require complex organic molecules to grow.
• tend to grow in acidic conditions.
• can be divided into yeasts and molds.
• are mostly saprophytes (living on dead matter).
• are eukaryotes.

Fungi are all eukaryotic require complex organic molecules to grow and are referred to as chemoheterotrophs. They play a major role in the breakdown of organic matter, making acid products. The cells of fungi have membrane-bound nuclei and organelles and so fungi are eukaryotic. Yeasts are unicellular fungi that multiply by a process of “budding”: molds are filamentous fungi. Many fungi are microscopic, but molds can grow to be very large, as with bracket fungi, mushrooms, toadstools and puffballs. Fungi have cell walls of a different structure from the cell walls of either bacteria or plants. Many fungi live on dead organic matter and are thus saprophytes.

Question 20

The lipopolysaccharide of the outer membrane of Gram-negative bacteria can be toxic and is known as:

Answer 20

• LPS
• endotoxin

A toxoid is a toxin that has lost its biological activity, but that retains its antigenic structure, stimulating the production of antibodies that can protect against the activity of the native toxin. The genus Toxoplasma includes Toxoplasma gondi, a protist parasite. Exotoxins are proteins secreted by bacteria, and harmful to higher organisms. The abbreviation often used for lipopolysaccharide is LPS. This can cause the symptoms of Gram-negative shock, including elevated body temperature, low blood pressure and rapid heartbeat, and it is sometimes referred to as endotoxin as a consequence.

Question 21

Certain fungal species or structures may be described as:

Answer 21

• mesophiles
• psychrophiles
• geotropic
• thermophiles

Fungi are not photosynthetic, requiring a supply of organic carbon for growth. Most fungi grow at moderate temperatures as mesophiles, growing at temperatures between 10 and 40°C, with optima between 25 and 35°C, and this group includes fungal pathogens of mammals. Fungal psychrophiles grow at temperatures below 20°, and sometimes below 0°C. They may contaminate meat stored in refrigerators. Thermophilic fungi are generally considered to grow at temperatures ranging from 20 to 50°C and with optima at or near 40°C and a maximum growth temperature of between 60 and 62°C. They play an important role during composting of plant waste material. Reproductive structures such as basidiocarps and sporangiophores may exhibit positive photropism - growth towards the light, and/or negative geotropism - growth away from the center of the earth due to the influence of the force of gravity. Do not confuse PHOTOTROPISM - movement towards the light with PHOTOTROPISM - using light as a source of metabolic energy.

Question 22

Viroids are:

Answer 22

• small, circular single stranded RNA molecules.

Question 23

Viruses are identified as a separate group of microorganisms because:

Answer 23

• they contain only a single type of nucleic acid in the virion.
• of their mechanism of replication.

Viruses are obligate intracellular parasites with a characteristically simple organization and mode of replication although the chlamydia is also obligate intracellular parasites. They do not possess cell walls or nuclei, and cannot thus be described as eukaryotic. Being akaryotic, they do not have a cellular structure and thus cannot be one of the prokaryotes. The term “akaryote” may be misleading - it simply means “lacking a nucleus”. Thus, erythrocytes may be described as akaryotic. It is debatable whether viruses are even truly alive.

Question 24

The Ziehl-Neelsen (ZN) stain is used:

Answer 24

• to visualize Mycobacterium leprae.
• in the presumptive diagnosis of tuberculosis.
• to visualize Mycobacterium tuberculosis.

Mycobacterium tuberculosis, the causative agent of tuberculosis, and Mycobacterium leprae, the cause of leprosy, are among a limited number of pathogens that are difficult to stain and are acid- alcohol-fast by stains such as the Ziehl Neelsen stain. This stain is now often replaced by the rhodamine/auramine fluorescent stain.

Question 25

The bacterial flagellum:

Answer 25

• is a rigid helical structure, producing motility by rotation.
• is composed of a protein called flagellin.

Bacterial flagella (singular - flagellum) are unlike the more complex eukaryotic cilia and flagella, which may have wave-like or whip-like activity. Flagella propel bacteria by rotatory motion. They are composed of protein and act as antigenic structures. Not all bacteria produce flagella, not even all motile bacteria. Bacteria without flagella may demonstrate “gliding motility”.

Question 26

Bacterial pili or fimbriae:

Answer 26

• may be associated with the transfer of DNA by conjugation in some bacteria in the case of certain specialized pili.
• may be involved in adhesion of the cells to surfaces.
• are exterior protein filaments.

It is bacterial flagella that are responsible for motility. Pili (singular - pilus) or fimbriae (singular - fimbria) are associated with adhesions to surfaces. This may help in the establishment of a bacterial infection. Sex pili join donor and recipient cells together during bacterial conjugation.

Question 27

Fungi may utilize the following as a source of carbon and energy:

Answer 27

• some pesticides.
• cellulose
• some plastics.
• methane
• hydrocarbons in machine oils.

It is probably safe to say that there is no naturally occurring organic material that cannot be used by one fungus or another. On the other hand, this doesn’t mean that all fungi can utilize all organic materials, e.g. the simplest of organic materials, methane, can only be utilized by a few yeasts.

Question 28

Fungal spores:

Answer 28

• may be motile or non-motile.
• that are asexual and designed for airborne dissemination are called conidia.
• may be produced within macroscopic fruiting structures.
• may be produced asexually or sexually.

Some fungal spores may be multicellular, such as macroconidia produced by Alternaria spp. Zoospores are motile.

Question 29

FACULTATIVE anaerobes include:

Answer 29

• Staphylococcus aureus.
• Escherichia coli.

Bacteroides fragilis and Clostridium botulinum are obligate anaerobes; Pseudomonas aeruginosa is an obligate aerobe, although it may grow without oxygen if nitrate is present due to anaerobic respiration.

Question 30

RNA dependent DNA polymerase catalyzes:

Answer 30

• the production of DNA from an RNA template.

RNA dependent RNA polymerase (reverse transcriptase) catalyzes the production of DNA from an RNA template. It is a very important enzyme associated with retroviruses such as the Human Immunodeficiency Virus (HIV), the cause of AIDS.

Question 31

Structures lying outside the bacterial cell wall include:

Answer 31

• a capsule
• pili
• the Gram-negative outer membrane.
• fimbriae

Mesosomes are invaginations of the cell membrane and thus lie within the cell wall. All the other structures lie outside cell walls. Capsules help to protect bacteria from phagocytosis, and fimbriae help bacteria to adhere to surfaces. Pili are involved in bacterial gene transfer, and the Gram-negative outer membrane acts as endotoxin.