Module 4: Microbial Populations

Question 1

True or False

Bacteria are ubiquitous in every part of the world. Multitude of bacteria
are found in every environment, from mountaintops to the ocean floor, freezing temperatures to bubbling hot springs and from plant and animal bodies to forest soils. Each bacterium is adapted to live in a particular niche such as oceanic surfaces, mud sediments, soil or on the surfaces of another organism, in the air, uncontaminated natural bodies of water where bacterial population counts can be calculated in thousands or billions per gram.

Answer 1

True

Question 2

The time required for the formation of a generation.

Answer 2

Generation time

Question 3

True or False

Experts have devised a way to calculate generation time in
different species of bacteria. The generation time which varies among bacteria is controlled by many environmental conditions and by the nature of the bacterial species. One of the fastest-growing bacteria Clostridium perfringens has an
optimum generation time of about 10 minutes; Escherichia coli can double every 20 minutes; and the slow-growing Mycobacterium tuberculosis has a generation
time in the range of 12 to 16 hours. Some researchers have suggested that certain bacterial populations living deep below the earth surface may grow at extremely slow rates, reproducing once every several thousand years.

Answer 3

True

Question 4

What are the 4 distinct sequential phases of bacterial growth that form the standard growth curve of bacteria?

Answer 4

Lag phase
Log phase
Stationery phase
Phase of decline

Question 5

It is the time when bacteria try to adapt to a new environment. There
is inability of the bacterial population to double the initial inoculum size thus the population remains temporarily unchanged. The bacteria may be deficient in enzymes and co-enzymes that support bacterial metabolism and cell division.

Answer 5

Lag phase

Question 6

A period of maximal cell growth. Cells divide steadily at a constant rate. At this stage the chemical composition of cells, their metabolic activity and other physiological characteristics of the population are uniform.

Answer 6

Exponential or Log phase

Question 7

It is characterized by a gradual tapering off of the log phase. A
trend towards the cessation of bacterial growth is evident and is attributed to a variety of factors. These may include exhaustion of some nutrients initially contained in the medium and the production and accumulation of toxic products that are detrimental to bacterial growth. The population remains constant for a time as a result of the possible cessation of cell division.

Answer 7

Stationary phase

Question 8

Characterized by a faster death rate than cell division. Bacterial death occurring at a faster rate is believed to be influenced by the depletion of essential nutrients and the accumulation of inhibitory products
such as acids. Phase of readjustment is the period when bacteria are
transferred and adapted to a new environment.

Answer 8

Phase of decline

Question 9

Bacteria differ dramatically with respect to the conditions that are
necessary for their optimal growth. In terms of nutritional needs, all cells require sources of carbon, nitrogen, sulfur, phosphorus, numerous inorganic salts (potassium, magnesium, sodium, calcium and iron) and a large number of other
elements called micronutrients (zinc, copper, manganese, selenium and molybdenum). Carbon is an element required in great amount by bacteria which is an important prerequisite for bacterial growth.

Answer 9

Nutritional requirements

Question 10

The optimal requirements for bacterial growth vary dramatically for different bacterial types. These requirements include oxygen, temperature, pH, osmotic pressure, and radiation.

Answer 10

The physical requirements

Question 11

This is a simple process whereby a cell divides to double its starting size and then split in two producing 2 daughter cells. For the dividing bacterial cell to remain viable and
competitive, cell division has to take place at the right time, in the right place, and a provision of a complete copy of the essential genetic material has to be given to each daughter cells.

Answer 11

Binary fission

Question 12

True or False

Before binary fission takes place, the genetic material (DNA) is copied and
copies are segregated on opposite ends of the cell. Many types of proteins that comprise the cell division machinery assemble at the future division site. A key component of this machinery is the protein FtsZ. Protein monomers of FtsZ assemble into a ring-like structure at the center of a cell while components of
the division apparatus assemble at the FtsZ ring. This machinery is positioned so that division splits the cytoplasm but does not damage DNA in the process. As division occurs, the cytoplasm is cleaved in two and a new cell wall is synthesized. The order and timing of these processes (DNA replication, DNA segregation, division site selection, invagination of the cell envelope and synthesis of new cell wall) are tightly controlled.

Answer 12

True

Question 13

There are groups of bacteria (Pleurocapsales, an Order of Cyanobacteria) that undergo unusual forms or patterns of cell division to reproduce. The process starts out with the formation of a small, spherical cell approximately 1 to 2 µm in diameter referred to as a baeocyte (which literally means “small cell”). The baeocyte begins to grow, eventually forming a vegetative cell up to 30 µm in diameter. As it grows, the cellular DNA is replicated over and over, and the cell produces a thick extracellular material. The vegetative cell eventually transforms
into a reproductive phase where it undergoes a rapid succession of cytoplasmic fissions to produce dozens or even hundreds of baeocytes or multiple number of
daughter cells.

Answer 13

Baeocyte production

Question 14

Reportedly been observed in some members of the Planctomycetes,
Cyanobacteria, Firmicutes (or the low G+C Gram-positive bacteria) and the
Proteobacteria. Although budding has been extensively studied in the eukaryotic yeast Saccharomyces cerevisiae, the molecular mechanisms of bud formation in
bacteria are not known.

Answer 14

Budding

Question 15

This is a quick and relatively easy way of estimating the density of
microbial populations. The microbes in a volume of bacterial suspension is
counted with the use of a slide (Petroff-Houser chamber).

Answer 15

Direct measurement of microbial growth

Question 16

These methods are used to measure microbial populations. These procedures involve a stepwise dilution of sample to reduce microbial load into quantifiable numbers before plating the sample in a suitable medium to allow the growth of colony forming units (CFU). The colonies that form on the agar
are counted and calculated to get an estimate of the number of cells per mL or gram of a sample. A surface viable count uses a micro-drop of sample on agar surface which allows the analysis of many samples in a single agar plate. This requires precision and accuracy in the pipetting and drying of agar surfaces before and after inoculation of samples.

Answer 16

Viable cell count by serial dilution and plate counting

Question 17

These methods use spectrophotometer and MacFarland standards to visually estimate the cell density of bacterium. Each MacFarland scale represents a specific concentration (Colony forming units/mL).

Answer 17

Turbidimetric analyses

Question 18

In the initial stage of a latent
period, host cells do not contain any infective virion.

Answer 18

Eclipse phase

Question 19

The number of virions per unit volume.

Answer 19

Viral titer

Question 20

The number of virions released
from the host cell.

Answer 20

Burst size

Question 21

It involves the formation of virions during the infection process in target host cells. Viruses first gain entry into the host cells before viral replication can occur. Viral replication varies
among viruses and this depends on the type of genes that regulate replication among them. DNA viruses tend to assemble in the nucleus while most RNA viruses develop in the cytoplasm.

Answer 21

Viral replication

Question 22

5 stages where vir replication occurs.

Answer 22

Adsorption
Penetration and uncoating
Replication and biosynthesis
Virion assembly
Viral release

Question 23

This is the first step of viral replication. This involves attachment of a viral particle to the cell membrane of susceptible host’s cell (by ionic bonding or changes) or by attachment with molecular entities on the host’s cell surfaces which act as receptors. It is claimed that this requires interactions of host cells
with proteins on the capsids of naked virions or envelopes of enveloped virions.

Answer 23

Adsorption

Question 24

The cell membrane of the host cell invaginates the virus particle enclosing it in a pinocytotic vacuole that protects the virion from antibodies of the host. Some experts see that the enclosure of the virion in the pinocytic vacuole leads to the engulfment of the whole virion by the host cell, stripping off of the viral protein coat or capsid (uncoating) as a function of lysosomal enzymes. This is followed by the release of nucleic acid or genome into the cytoplasm of host cell.

Answer 24

Penetration and uncoating

Question 25

These processes take place in the viral nucleus or cytoplasm. The virus
takes advantage of the existing cell structures to replicate itself. For viruses, the infecting RNA produces messenger RNA (mRNA) which is used to instruct the host cell to make viral components. m-RNA codes for early enzyme formation
required for nucleic acid replication (transcription), translation of the genome into protein products and for proteins that inhibit synthesis of other cellular macromolecules such as capsid and envelope.

Answer 25

Replication and biosynthesis

Question 26

The newly synthesized genome (nucleic acid) and proteins are assembled to form new viral particles or virions (active or intact viral particle). Viral assembly may take place in the cell nucleus or cytoplasm for most developed viruses.

Answer 26

Viral assembly

Question 27

The mature viruses are released from host cells in different ways. Release
is possibly made by sudden rupture of the cell (death), gradual extrusion (force out) of enveloped viruses before invading or attacking other host cells, may remain dormant in the cell, while some animal viruses may also be released from host cells that do not undergo cell lysis. Other viruses may leave infected host cells by budding through the host cell membrane without directly killing the cell.

Answer 27

Viral release

Question 28

True or False

Yeasts exhibits the classical S-shaped curve when grown in vitro. A pattern
of growth in which the population density of the organism increases slowly initially in a positive acceleration phase in an environment. This slow pattern of
growth increases rapidly approaching an exponential growth rate (as in a J-
shaped curve) but then a point is reached when the upward curve begins to level
off which reflects increasing environmental resistance.

Answer 28

True

Question 29

True or False

Exponential growth may be possible when unlimited environmental
resources are available but this is not the case in the real microbial world. It was recognized by scientists like Charles Darwin that this fact is congruent with struggle for existence, which states that individuals compete (with members of their own or other species) for limited resources. The successful ones tend to survive to pass on their characteristics and traits (which are possibly transferred
by genes) to the next generation at a greater rate (natural selection process).

In the real microbial world, exponential growth may occur quickly in environments where there are few microbes and plentiful resources, but as the number of microbes increases, resources become depleted which decelerates rate of microbial growth abruptly (phase of decline and microbial death).

Answer 29

True