NTA Level 5 • Semester 1 • PST05103

Internal structures and drug targets in prokaryotic cells

Pharmaceutical Microbiology • Source Session/Topic 4

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Session 4: Internal structures and drug targets in prokaryotic cells

Total Session Time: 120 minutes

Pre-requisites

• Human anatomy and physiology

Students Learning Tasks

By the end of this session students are expected to be able to:

• Describe the structure of a bacterial cell

• List functions of different bacterial organelles/structures

• Describe various drug targets in the bacterial cell

Resources Needed:

• Flip charts, marker pens, and masking tape

• Black/white board and chalk/whiteboard markers

SESSION OVERVIEW

SESSION CONTENTS

STEP 1: Presentation of Session Title and Learning Tasks (5 minutes)

READ or ASK students to read the learning tasks and clarify

ASK students if they have any questions before continuing

STEP 2: Internal Structures of Prokaryotic Cells (80 minutes)

• Bacteria are one group of prokaryotes, the other group is the archaea

• Bacterial structure can be described in terms of external and internal

structures

• Internal structures of bacterial cells are those internal to the cell

wall

• The bacterial cell wall and its associated structures comprise the

external structures of the bacterial cell

|Activity: Small Group Discussion (40 minutes) |

| |

|DIVIDE students into small manageable groups |

| |

|ASK students to discuss on the following question |

|Draw a labeled diagram showing internal structures of a bacterial |

|cell |

|List the function(s) of each of the labeled structure |

| |

|ALLOW students to discuss for 30 minutes |

| |

|ALLOW few groups to present and the rest to add points not |

|mentioned |

| |

|CLARIFY and SUMMARIZE by using the contents below |

• The internal structure of a prokaryotic cell consists of the cytoplasm

which contains enzymes needed for metabolic reactions.

[pic]

Source: https://www.slideshare.net/mdsalaadx1/anatomy-and-physiology-

of-bacteria

The Cytoplasm

• The cytoplasm is an aqueous thick fluid enclosed by the cell membrane

• It is made up of 80% water, and proteins (enzymes), carbohydrates,

lipids, inorganic ions and many low-molecular-weight compounds

• Fills the cell and it is where most cellular reactions and activities

take place

• It is the internal environment of the cell

The cell membrane

• The cell membrane is located immediately beneath the cell wall

• The cell membrane is a thin membrane around the cytoplasm

• The cell membrane is also called the cytoplasmic membrane or plasma

membrane

• It contains phospholipids, proteins and polysaccharides

• The cell membrane is a vital structure and critical barrier that

separates the inside of the cell from the outer environment

• The gram positive bacteria have one plasma membrane while the gram

negative bacteria have two plasma membranes –one is internal and the

second is known as an outer membrane separated from the inner membrane

by the periplasmic space

Figure 4.1: Cell membrane in Gram positive and Gram negative bacteria

[pic]

• Functions of plasma membrane;

o The plasma membrane retains the cytoplasm, particularly in cells

without cell walls, and separates it from the surroundings

o Plasma membranes serve as a selectively permeable barrier; it allows

particular ions and molecules to pass, either into or out of the cell,

while preventing the movement of others

o Transport systems can be used for such tasks as nutrient uptake, waste

excretion, and protein secretion

o The plasma membrane also is the location of a variety of crucial

metabolic processes; respiration, photosynthesis, the synthesis of

lipids and cell wall constituents, and probably chromosome

segregation.

Mesosomes

• In gram positive bacteria, the plasma membrane forms in folding

• The infolding form structures called mesosomes within cytoplasm

• The mesosomes are associated with bacterial nuclear material and its

replication

• Respiratory enzymes are also associated with mesosomes

• Functions of mesosomes

o They are imvolved in cell wall formation during cell division.

o They play a role in replication of chromosome and distribution to

daughter cells.

o They are also involved in secretary processes

Nucleoid

• Nucleoid is an area within the cytoplasm of a prokaryotic cell where

DNA is located

• It is equivalent to the nucleus in eukaryotic cells

• It is not enclosed in a membrane i.e. no nuclear membrane (not a true

nucleus) but it is visibly separate from the rest of the cell

interior.

• The bacterial DNA is single-stranded, circular and haploid

Plastids

• Prokaryotic cells do not have mitochondria or chloroplast

o The electron transport enzymes are located in the cytoplasmic membrane

o Photosynthetic pigments (Carotenoids, bacterial chlorophyll) of

photosynthetic bacteria are located below the cell membrane

o In some Cyanobacteria (blue-green algae), the photosynthetic

structures are called Thylakoids

o Thylakoids contain light harvesting pigments called phycobilins

o Plastids provide energy for the prokaryotic cell as well as carrying

out photosynthesis process in cyanobacteria

Inclusion or storage granules

• Inclusion granules are storage structures within the bacterial

cytoplasm

• There are several types of storage granules for nutrients in

prokaryotic cells

o i.e. they serve as energy and nutrient reservoirs

• Nutrients may be stored in the cytoplasm in the form of lipids,

glycogen, polyphosphate (and sometimes sulphur (volutin granules) or

nitrogen

• When source of nitrogen, sulphur, or phosphorus is limited or when pH

is low, excess CO2 in the medium may be converted to “Poly-β-

hydroxybutric acid, starch and glycogen which are used as energy

sources

Endospores

• Some bacteria (e.g. Clostridium) form spores that are very resistant

to drought, high temperatures and other hazards of nature. Once the

hazard is removed, the spores germinate to form new bacteria

• Spores or endospores are survival structures for some bacteria

• Endospores are formed by a process of sporogenesis which involves

duplication of DNA and cytoplasm cleavage to form two structures that

are then covered by a tough layer and freed from the cell

Ribosomes

• They are structures made up of both protein and ribonucleic acid (RNA)

• Ribosomes are made up of two subunits, one larger and the other

smaller

• They translate the genetic message in messenger RNA (mRNA) so as to

produce proteins

• They are the sites for protein synthesis

• Bacterial ribosomes occur freely in cytoplasm and in clusters

• The clusters are called the polysomes

• Bacterial ribosomes are of 70S consisting of 50S and 80S subunits

(smaller than those of eukaryotic cells)

▪ The larger eukaryotic ribosomes have are 80S with 60S and 40S

subunits

Plasmids

• Plasmids are non-chromosomal genetic material.

• They are extrachromosomal, double-stranded, circular DNA molecules

capable of replicating independently of the bacterial chromosome and

are smaller in size

o May be integrated into the bacterial chromosome

• Are much smaller than nucleoid DNA

• Plasmid DNA is used for gene manipulation in biotechnology.

• Are very important as may carry genes for antibiotic drug resistance

• Because plasmids move between different bacteria, then it is

responsible for drug resistance

• Plasmids code for other important materials in the prokaryotic cell;

o Bacteriocins:

▪ Bacteriocin are toxins or enzymes produced by certain bacteria and

are lethal (i.e. kill) to other bacteria

o Antibiotics:

▪ Substances produced by Streptomyces e.g. Streptomycin from

Streptomyces griseus

▪ They inhibit or kill other microorganisms

o Degradative enzymes:

▪ They are especially produced Pseudomonas

▪ Such bacteria (Pseudomonas) produce powerful enzymes capable of

cleaning up environmental hazards such as oil spills and toxic

chemical wastes by degrading them into harmless products

o Tumours caused by Agabacterium in plants

o Nitrogen fixing enzymes in Rhizobium in the root nodules of legumes

which help nourishing the soil

[pic]

Source: https://nptel.ac.in/courses/102103015/module2/lec2/2.html

Transposons

• Transposons are pieces/sequences of DNA that move rapidly from one

site to another, within or between DNAs of bacteria, plasmids and

bacteriophages

• They are also known as “jumping genes” and the movement is known as

transposition

• They code for drug resistance enzymes, toxins or metabolic enzymes and

cause mutations in genes alter other genes

STEP 3: Drug Targets in Bacterial Internal Structures (25 minutes)

• The following are some drug targets in internal structures of bacteria

• Ribosomes

o Bacterial ribosomes are smaller in size than eukaryotic ribosomes

o The smaller bacterial ribosomes (70S) have 50S and 30S subunits

o The difference in size between these ribosomes provides targets for

antimicrobial drugs

o These drugs include

▪ Chloramphenicol and macrolides which bind to 50S subunit

▪ Tetracyclines and Aminoglycosides bind to 30S subunit

• DNA Synthesis

o DNA gyrase and topoisomerase IV

▪ Inhibited by Fluoroquinolones (e.g. Ciprofloxacin, Moxifloxacin

etc.)

▪ Nitroimidazoles e.g. Metronidazole

• RNA synthesis (RNA polymerase)

o Rifampicin

• Cell membrane

o Daptomycin disrupt structure of cell membranes

o Polymyxins (colistins)

STEP 4: Key Points (5 minutes)

• The internal structures of bacteria include all structures found

inside to the cell wall

• These structures are distributed within the cytoplasm which a semi

fluid internal environment of the cell enclosed in a cell membrane

• The cell membrane is made up of lipids and proteins and its function

is to enclose the cytoplasm

• Ribosomes are for protein synthesis

• Inclusion granules are for storage of materials in the cell

• Structures that provide targets for antibacterial drugs include the

cell membrane, ribosomes and the DNA

• Gram negative have two cell membranes, inner cell membrane and the

outer cell membrane

STEP 5: Evaluation (5 minutes)

• List the internal structures of a bacterial cell

• Name the drug targets among the internal structures of bacteria

• What are the functions of the nucleoid in bacteria?

References

Hugo and Russell (2011), Pharmaceutical Microbiology 8th Edition, Willey-

Blackwel publications

Karen C. Carroll et al (2013); Jawetz, Melnick and Adelberg’s Medical

Microbiology 26th Ed. McGraw Hill Co. Inc.

Greenwood et al (2012); Medical Microbiology, 18th edition Churchill

Livingstone

Jeffery C. Pommerville. Alcamo's Fundamentals of Microbiology (Tenth

Edition). Jones and Bartlett Student edition

Gerard J. Tortora, Berdell R. Funke, Christine L. Case. Pearson –

Microbiology: An Introduction. Benjamin Cummings.

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