Intracellular bacteria

Intracellular bacteria are pathogens that survive and replicate within the host cells. They are categorized into obligate intracellular and facultative intracellular bacteria based on their dependency on the host cell for survival and replication.

1. Obligate Intracellular Bacteria

These bacteria rely entirely on the host’s cellular machinery, including ATP, for survival and replication. They cannot live outside host cells.

Examples:

Rickettsia
Chlamydia
Coxiella

Mnemonic:

“Stay inside (cells) when it is Really Chilly and Cold”:

Rickettsia
Chlamydia
Coxiella

Characteristics:

Dependent on host ATP for energy.
Must remain inside host cells to survive and replicate.

2. Facultative Intracellular Bacteria

These bacteria can live both inside and outside the host cells, meaning they are not completely dependent on the host cell for survival but often reside within cells for protection from the host immune system.

Examples:

Salmonella
Neisseria
Brucella
Mycobacterium
Listeria
Francisella
Legionella
Yersinia pestis

Mnemonic:

“Some Nasty Bugs May Live FacultativeLY”:

Salmonella
Neisseria
Brucella
Mycobacterium
Listeria
Francisella
Legionella
Yersinia pestis

Characteristics:

Can survive both inside and outside host cells.
Use intracellular life to evade immune responses.

Table: Comparison of Obligate and Facultative Intracellular Bacteria

Feature	Obligate Intracellular	Facultative Intracellular
Examples	Rickettsia, Chlamydia, Coxiella	Salmonella, Neisseria, Brucella, Mycobacterium, Listeria, Francisella, Legionella, Yersinia pestis
Dependency on Host Cell	Complete reliance on host cell machinery	Can survive both inside and outside host cells
Energy Source	Host-derived ATP	Can generate their own ATP outside host cells
Infection Strategy	Must remain inside host cells	Can switch between intracellular and extracellular environments

Table: Comparison of Obligate and Facultative Intracellular Bacteria

Points to Remember:

Obligate intracellular bacteria are fully dependent on host cells for survival and replication.
Facultative intracellular bacteria can survive in both intracellular and extracellular environments, giving them flexibility in evading the host immune system.
The obligate intracellular group includes organisms like Rickettsia, Chlamydia, and Coxiella.
The facultative intracellular group includes a wider range of pathogens like Salmonella, Neisseria, and Mycobacterium.

Bibliography:

Todar, K. (2020). Pathogenic Bacteria. Todar’s Online Textbook of Bacteriology.
Ryan, K.J., & Ray, C.G. (2004). Sherris Medical Microbiology (4th ed.). McGraw-Hill.

USMLE Step 1 Review Series

Curriculum

Intracellular bacteria

1. Obligate Intracellular Bacteria

Examples:

Mnemonic:

Characteristics:

2. Facultative Intracellular Bacteria

Examples:

Mnemonic:

Characteristics:

Table: Comparison of Obligate and Facultative Intracellular Bacteria

Points to Remember:

Bibliography:

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