Microorganisms

Viruses (Acellular Agents)

are obligate intracellular parasites. They are not true cells; they consist merely of genetic material wrapped in a protein coat. They must hijack a host cell's machinery to replicate.

Virus: genetic material

Contain either DNA or RNA, never both.

Virus: PHLE Rule of Thumb (Exceptions are highly tested)

• All DNA viruses are Double-Stranded (dsDNA) EXCEPT Parvovirus (ssDNA).

• All RNA viruses are Single-Stranded (ssRNA) EXCEPT Reovirus/Rotavirus (dsRNA).

Capsid

The protein shell protecting the genome (usually helical or icosahedral).

Envelope

A lipid bilayer acquired when the virus buds out of the host cell.

PHLE Trap: Naked (non-enveloped) viruses

(e.g., Norovirus, Poliovirus, Hepatitis A) are more resistant to alcohol-based sanitizers, heat, and stomach acid than enveloped viruses (e.g., HIV, Influenza, SARS-CoV-2). Envelopes are easily destroyed by lipid solvents (soap, alcohol).

Fungi (Eukaryotes)

are chemoheterotrophs that absorb nutrients from their environment. They are a massive topic in pharmacology because their unique cell wall and membrane provide distinct drug targets

Fungi: Pharmacological Targets

• Cell Membrane: Contains Ergosterol (Targeted by Azoles like Fluconazole, and Polyenes like Amphotericin B).

• Cell Wall: Contains Chitin and $\beta$-Glucans (Targeted by Echinocandins like Caspofungin).

Fungi: Morphological Classification

Yeasts

Molds

Dimorphic Fungi

Yeasts

Unicellular, reproduce by budding (Candida albicans, Cryptococcus neoformans).

Molds

Multicellular, consist of branching filaments called hyphae (Aspergillus, Dermatophytes).

Dimorphic Fungi

Can exist as either yeast or mold depending on temperature

Morphological Classification: board mnemonic

"Mold in the Cold (20 deg), Yeast in the Beast (37 deg / inside the human body)."

Examples: Histoplasma, Blastomyces, Coccidioides.

Parasites (Eukaryotes)

Clinical parasitology is divided into two major groups based on size and structure.

• Protozoa

• Helminths

Protozoa

Microscopic, unicellular eukaryotes.

Amoebas

Move by pseudopods (Entamoeba histolytica - dysentery).

Flagellates

Move by flagella (Giardia lamblia, Trichomonas vaginalis).

Sporozoans (Apicomplexa)

Non-motile obligate intracellular parasites (Plasmodium species causing Malaria, Toxoplasma gondii).

Helminths

Macroscopic, multicellular parasitic worms.

Prions (Proteinaceous Infectious Particles)

are entirely unique infectious agents that contain no nucleic acid (no DNA, no RNA).

Prions: mechamism

They are abnormally folded proteins (PrP^Sc) that induce normal brain proteins (PrP^C) to misfold, leading to massive brain cell death.

Prions: Clinical Presentation

Cause Transmissible Spongiform Encephalopathies (TSEs). The brain becomes spongy and full of holes.

• Human diseases: Creutzfeldt-Jakob disease (CJD), Kuru.

• Animal diseases: Bovine Spongiform Encephalopathy (Mad Cow Disease), Scrapie (sheep).

Prions: PHLE Manufacturing Note

Prions are highly resistant to standard autoclaving (121 deg for 15 mins), UV radiation, and boiling. Decontamination requires extreme measures (e.g., 1 N NaOH followed by extended autoclaving at 134 deg).