Microbiology: USMLE Step 1 video lessons
Bacterial structure, toxins and genetics, antibiotic resistance, viruses, fungi and the organisms behind classic infections.
Bacterial cell walls and stains, endotoxins and exotoxins, the clostridia, staphylococci and streptococci, encapsulated organisms, bacterial genetics, antibiotic resistance, virus replication, HIV, hepatitis B serology, antifungal drugs and bacterial meningitis by age.
5:39Gram-positive vs gram-negative cell walls
The Gram stain looks like a dye test, but it is a wall test. Compare the two cell envelopes and use them to predict the stain and antibiotic susceptibility.
5:01Acid-fast stain and the mycobacterial cell wall
Mycobacteria wrap their wall in wax. That one feature explains why the acid-fast stain works, why Gram staining does not, and why many antibiotics struggle to get in.
5:46Endotoxin vs exotoxin: lipid A, A-B toxins and toxoids
Endotoxin is a piece of the gram-negative outer membrane. An exotoxin is a protein that living bacteria secrete. Both can cause shock, but they differ in source, chemistry, mechanism and effect.
5:54Bacterial toxins that raise cAMP: cholera, pertussis, anthrax
Four toxins from four bacteria all push cyclic AMP up, but each takes a different route. Knowing the route, and the cell it acts on, predicts the clinical picture.
5:22Clostridial toxins compared: tetanus, botulinum, alpha, C. difficile
Tetanus and botulinum toxins cut the same SNARE proteins but give opposite pictures. Compare the clostridial toxins by their target and their clinical picture.
5:23Staph vs strep: catalase, coagulase and haemolysis
Two tests do most of the work in staph vs strep identification. Catalase separates the genera, then each genus has its own fork: coagulase and novobiocin, or haemolysis and a susceptibility disc.
5:08Encapsulated organisms and the spleen: asplenia and conjugate vaccines
A missing spleen turns a minor illness into an emergency because encapsulated bacteria depend on it for clearance. Conjugate vaccines work by bringing in T cell help.
5:13Bacterial genetics: how bacteria swap genes
Bacteria divide by binary fission, yet genes still move sideways between cells. Each route of horizontal gene transfer has one identifying clue.
5:13Beta-lactamase and antibiotic resistance: three ways bacteria win
Bacteria resist beta-lactams by destroying the drug, changing its target or keeping it out. Only the first can be beaten by a beta-lactamase inhibitor such as clavulanate.
5:48DNA vs RNA virus replication: which enzyme each genome needs
Where DNA and RNA viruses replicate, what positive-sense and negative-sense mean, and the one rule that tells you which enzyme a virion must carry.
5:39HIV life cycle and drug targets, step by step
Six steps take HIV from attachment to a mature virion, and antiretroviral drug classes block them. Learn the order, the drugs and why the integrated provirus cannot be cleared.
5:17Hepatitis B serology: reading the panel
A hepatitis B panel is built from three viral antigens and their antibodies. Learn what each marker means and how the combinations separate acute, chronic, recovered and vaccinated patients.
5:26Antifungal drugs explained: mechanisms of action
Fungi are eukaryotes like us, so antifungal drugs must hit the few things human cells lack. Classify the five classes by the fungal target each one inhibits.
5:19Bacterial meningitis organisms by age and risk group
The cerebrospinal fluid tells you the meningitis is bacterial, but not which bacterium. Age and risk group predict the organism, and the Gram stain confirms it.