General Microbiology (Micr300)

Lecture 9Microbial Diversity: Fungi &

Viruses

(Text Chapter: 9.1; 9.2; 9.5-9.13; 14.12)

Quiz #2 will be held on Monday May 1

Fungi

Fungi are widely distributed and are found wherever moisture is present.

There are two major types of fungi, yeasts and molds.

Yeast

A yeast is a unicellular fungus that has a single nucleus and reproduces either asexually by budding and transverse division or sexually through spore formation.

Molds

A mold consists of long, branched, threadlike filaments of cells called hyphae that form a mycelium.

Dimorphic Fungi

Many fungi, especially those that cause diseases in humans and animals, are dimorphic - that is, they could have either yeast or mold forms, depending on the changes in environmental conditions. This is called YM shift.

Fungal Structures

Fungal cell walls resemble plant cell walls architecturally but not chemically. Although the plant cell wall polysaccharide cellulose is present in the walls of certain fungi, most fungi contain chitin, a polymer of the glucose derivative N-acetylglucosamine, in their cell walls.

From the fungal mycelium, other hyphal branches may reach up into the air above the surface, and spores called conidia are formed on these aerial branches (Figure 14.30).

Mushrooms

Mushrooms are large, often edible fungi that produce fruiting bodies containing basidiospores (Figure 14.32).

General Microbiology (Micr300)

Microbial Diversity: Viruses

(Text Chapter: 9.1; 9.2; 9.5-9.13)

Viruses’ Uniqueness

Viruses have no cellular structure (acellular).

Viruses cannot reproduce unless they are inside a host cell.

A viral genome could be either DNA or RNA, but not both (the genomes of all other organisms are DNA).

Virus and Virion

Viruses are genetic elements that replicate independently of a cell’s chromosome(s) but are characterized by having an extracellular state (virion).

A virion is the extracellular form of a virus. The RNA or DNA genome may be double stranded or single stranded (Figure 9.1). After the virus genome is introduced into a new host cell the virus redirects the host metabolism to support virus replication.

Structure

Size: virions range between 10 - 400 nm in diameter;

Nucleocapsid: composed of a nucleic acid held within a protein coat called capsid. Capsid function:

Protects genome information aids in transfer between host cells

Structure

General shapes of virion structure: icosahedral viruses: 20 triangle faces

and 12 vertices helical viruses: cylinder shape enveloped viruses: have a membranous

layer surrounding the nucleocapsid complex viruses: neither icosahedral

nor helical

Structure

Nature of genomes. In almost all virions, genomes are either DNA

or RNA (except human cytomegalovirus, 1 DNA + 4 RNAs)

Very diverse: Single stranded DNA Double stranded DNA (most DNA viruses) Single stranded RNA (most RNA viruses) Double stranded RNA These could be linear or circular Segmented or nonsegmented

Viruses vs Cellular Organisms

Viruses are different from cellular organisms in the following aspects: They are obligate intracellular parasites They have no genes that encode the

proteins that function as the metabolic machinery for energy generation

They have no genes encoding proteins involved in protein synthesis

They may or may not contains genes that encode enzymes for nucleic acid synthesis

Viral Replication

The virus life cycle can be divided into five stages: attachment (adsorption), penetration (injection), uncoating (for some viruses entering cells via phagocytosis), protein and nucleic acid synthesis, assembly and packaging, and virion release (Figure 9.8).

These stages in virus replication begin when virions attach to host cells (Figure 9.9).

For some, uncoating is necessary

Host Restriction

Prokaryotes have specific DNA destruction systems that can destroy newly injected viral DNA.

The destruction is brought about by host restriction endonucleases, enzymes that cleave viral DNA at one or several places, thus preventing its replication - a phenomenon called restriction.

The host’s own DNA will not be affected due to specific modifications on the nucleotides around the sites where the endonucleases cut.

Plus and Minus Designation

By convention, mRNA is said to be in the plus (+) configuration. Its complement is said to be in the minus (–) configuration. This nomenclature is also used to describe the configuration of the genome of a single-stranded virus, whether its genome contains RNA or DNA.

For example, a virus that has a single-stranded RNA genome with the same orientation as its mRNA is said to be a positive-strand RNA virus. A virus whose single-stranded RNA genome is complementary to its mRNA is said to be a negative-strand RNA virus.

Viral Diversity: bacteriophages

Bacterial viruses (bacteriophages) are very diverse (Figure 9.12). The best-studied bacteriophages infect enteric bacteria such as Escherichia coli and are structurally quite complex, containing heads, tails, and other components.

Viral Diversity: animal viruses

Animal viruses include all known types of viral genomes (Figure 9.23).

Animal Viruses

Many animal viruses are enveloped, picking up portions of the host cytoplasmic membrane as they leave the cell.

Not all infections of animal host cells result in cell lysis or death; latent or persistent infections are common, and some animal viruses can cause cancer.

Retroviruses

Retroviruses are RNA viruses that replicate through a DNA intermediate. The retrovirus called human immunodeficiency virus (HIV) causes AIDS.

The retrovirus virion contains an enzyme, reverse transcriptase, that copies the information from its RNA genome into DNA, a process called reverse transcription.

Influenza (Flu) Viruses

Influenza viruses contain -RNA genome, with segmented linear ssRNA molecules

They can be divided into Groups A, B, C. Group A viruses are the cause of

epidemic flu seasons and can be further subtyped based on HA (hemagglutinin) and NA (neuraminidase) antigens

HA and NA are viral surface proteins important for attachment and virulence

A Diagram of Influenza Virion

Influenza (Flu) Viruses

Antigenic drift: accumulation of mutations in HA and NA antigens in a single strain, not very drastic.

If more than one such strains are co-infecting a single cell, re-assortment of these mutations among different strains may occur, resulting in a new strain – this is called Antigenic shift. Antigenic shift could cause major flu epidemic or pandemic.