Bacterial structure and functions

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Bacterial structure and functions


Introduction to bacteria

Bacterial structure and functions


Bacterial structure and functions

USMLE® Step 1 questions

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USMLE® Step 1 style questions USMLE

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A researcher is culturing bacteria on blood agar. The agar plate is shown below. Which of the following pathogens was most likely isolated?  

Image reproduced from Wikimedia Commons


Bacteria are prokaryotic cells that play an important role in human disease and health.

They can cause disease but are also part of the human microbiota and live on our skin, body and on everyday objects in our environment.

When compared to eukaryotic cells, the structure of bacteria is less complex due to a lack of nucleus and membrane-bound organelles such as mitochondria, endoplasmic reticulum and Golgi complexes.

Now, bacterial cells are often surrounded by several layers, which are collectively called the cell envelope.

Let’s start with the cell wall which is found on the outer surface of the cell membrane and its most important role is to protect the bacteria.

It consists of peptidoglycan which usually helps differentiate whether bacteria are Gram positive or Gram negative.

Gram positive bacteria have a single thick layer of peptidoglycan above the plasma membrane, which allows them to retain the staining dye, and Gram negative bacteria have a thinner layer of peptidoglycan sandwiched between the surface membrane and the plasma membrane, so they can’t retain the dye.

Additionally, the cell wall helps maintain their shape.

The round shaped bacteria are called cocci, the rod shaped ones are called bacilli, spiral shaped ones are spirilla, and sometimes the same bacteria can have multiple forms, in which case they’re called pleomorphic.

Some bacteria are covered by a capsule, which acts as a shield that protects the bacteria against phagocytosis, and also helps the bacteria adhere to surfaces.

The capsule is considered an important virulence factor since the strains that lack a capsule are less virulent.

Underneath the bacterial cell wall, there’s the plasma membrane which is the most important layer because it encloses the cytoplasm which is a gel-like substance composed mainly of water that also contains cell components, enzymes, and various organic molecules.

If the plasma membrane is removed, the cell’s contents spill into the environment and the cell no longer exists.

Now, the plasma membrane is responsible for most of the cell’s relationship with the outside world by acquiring nutrients and eliminating waste, and also maintains the interior of the bacteria in a constant, highly organized state.

Usually, all plasma membranes are selectively permeable barriers which allow certain ions and molecules to pass in and out of the cell, while preventing the movement of others.


Bacteria are prokaryotic, single-celled organisms that are found almost anywhere in the environment. Some are known to cause diseases, whereas others live as normal flora in different body parts such as the gut, skin, and genital organs.

Bacteria have cell walls for maintaining their shape and for protection, also from which we can determine whether they're Gram-positive or Gram-negative bacteria. Bacteria have another layer called the plasma membrane, located underneath the cell wall. The plasma membrane encloses the cytoplasm and plays important roles such as moving materials in and out of the cell, respiration, and photosynthesis. Inside the cytoplasm are ribosomes that synthesize proteins, the nucleoid which contains most of the bacteria's genetic material, and plasmids which contain genes that confer a selective advantage, such as antibiotic resistance.


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