Midterm


Important microscope parts



  • Base — foundation; used for carrying

  • Light source — located in base

  • Arm — used for carrying

  • Stage — supports the slide

  • Condenser — collects/directs light

  • Diaphragm — controls amount of light

  • Objective lenses — located on rotating nosepiece

  • Coarse focus knob — large knob; coarse focusing

  • Fine focus knob — smaller knob; fine focusing

  • Ocular lens — eyepiece






Resolution



Resolution = clarity/detail of an image.


More specifically, it is the smallest distance between two structures that still allows them to be seen as separate structures.


  • Best resolution with visible light: 0.2 µm

  • Resolution is limited by the wavelength of visible light.






2. 📏 Microbial Dimensions



Know the units:


  • Micrometer (µm) = one millionth of a meter

  • Nanometer (nm) = one billionth of a meter

  • Millimeter (mm) = one thousandth of a meter



General rule:


Virus → nm

Prokaryotes/bacteria → µm

Helminths → mm


The slides give a range from approximately 10 nm for viruses to 3–4 mm for protozoans.



Easy memory:



V-P-H = Virus → Prokaryote → Helminth

nm → µm → mm





3. 🟣 Simple Stains




Chromophore



The portion of a chromogen that gives it color.



Auxochrome



The charged portion of the chromogen that allows it to bind to the microorganism. This is what makes it a dye.



Basic stains listed



  • Methylene blue

  • Crystal violet

  • Safranin




Heat fixing



Heat fixing:


  • Kills bacteria

  • Adheres bacteria to the slide

  • Coagulates cytoplasmic proteins, making cells more visible

  • Can distort cells








4. 🟣 Gram Stain



The presentation shows the Gram-stain sequence:



Order:



1. Crystal violet → 2. Gram’s iodine → 3. Alcohol → 4. Safranin



What happens?



Crystal violet


  • Both Gram-positive and Gram-negative cells initially take up the stain.



Gram’s iodine


  • Helps form/trap the dye complex in the cell wall.



Alcohol


  • The critical decolorization step.

  • Gram-positive cells retain the crystal violet/iodine complex.

  • Gram-negative cells lose the primary stain.



Safranin


  • Counterstains the now-colorless Gram-negative cells.




Final appearance



Gram-positive = purple


Gram-negative = pink/red


The slides visually emphasize the difference between purple Gram-positive organisms and pink Gram-negative organisms.



HIGH-YIELD



If you see:


🟣 Purple → Gram-positive


🌸 Pink → Gram-negative





5. 🦠 Bacterial Shape & Arrangement




Shapes




Bacillus



Rod-shaped



Coccus



Spherical



Spiral forms



  • Vibrio

  • Spirillum

  • Spirochete



Also shown:


  • Star-shaped

  • Rectangular






Arrangements




Pairs



Diplo-


  • Diplococci

  • Diplobacilli




Chains



Strepto-


  • Streptococci

  • Streptobacilli




Clusters



Staphylococci



Groups of four



Tetrads



Cubelike groups of eight



Sarcinae



Memory trick:



  • Diplo = 2

  • Staphylo = cluster

  • Strepto = chain

  • Tetrad = 4

  • Sarcina = 8






6. 🪱 Trematodes — Flukes



The slides list:


  1. Clonorchis/Opisthorchis sinensis — Chinese liver fluke

  2. Paragonimus westermoni — lung fluke

  3. Schistosoma mansoni — blood fluke




Schistosoma mansoni



Important details:


  • Causes schistosomiasis

  • Associated regions: Brazil, Caribbean islands, Africa, Middle East

  • Transmission involves fecally contaminated water

  • Juveniles enter through the skin

  • Adults enter the colon and leave in feces

  • Diagnosis: eggs in feces

  • Symptoms:

    • Bloody diarrhea

    • Abdominal pain

    • Lethargy

    • Some people are asymptomatic




The slide specifically highlights the lateral structure of the egg.





7. 🪱 Cestodes — Tapeworms



The four listed groups/species are:


  1. Dipylidium caninum

  2. Echinococcus granulosus

  3. Hymenolepis (Vampirolepis) nana

  4. Taenia spp.



Taenia are parasitic tapeworms that can occur worldwide and can grow several meters long.





Taenia saginata — BEEF tapeworm




Transmission



Eating uncooked beef containing eggs or cysticerci.



Diagnosis



Requires identification of:


  • Scolex, OR

  • Gravid proglottid with 15–20 lateral branches




Symptoms



  • Mild nausea

  • Abdominal pain

  • Diarrhea

  • Headache






Taenia solium — PORK tapeworm




Transmission



Uncooked pork



Diagnosis



Scolex or gravid proglottid with 7–13 lateral branches



Important distinction



Cysticerci can infect tissues including:


  • Eyes

  • Brain

  • Heart

  • Liver

  • Lungs

  • Skeletal muscle



Symptoms depend on which tissues are involved.



Eggs



Both Taenia species have spherical eggs with a striated/radially spined shell.





⭐ Must memorize



Saginata = BEEF = 15–20 branches


Solium = PORK = 7–13 branches





8. 🪱 Nematodes — Roundworms



The slides list five:


  1. Ascaris lumbricoides

  2. Enterobius vermicularis

  3. Hookworms

    • Ancylostoma duodenale

    • Necator americanus


  5. Strongyloides stercoralis

  6. Wuchereria bancrofti



Roundworms are:


  • Unsegmented

  • Have a digestive tract

  • Have a body cavity

  • Have separate sexes








9. Ascaris lumbricoides




Important facts



  • Large — up to 50 cm

  • Causes ascariasis

  • Common in tropical regions

  • Eggs are ingested from infected soil

  • Juveniles leave intestinal walls and travel to the lungs

  • They are swallowed again

  • Adults develop in the small intestine

  • No intermediate host




Symptoms



Usually asymptomatic, but can develop into Ascaris pneumonia due to hypersensitivity and secondary bacterial infection.



Diagnosis



Eggs in feces


⭐ Look for the irregular edge of the egg.



Treatment



  • Antihelminthic drugs

  • Improved sanitation








10. Enterobius vermicularis — Pinworm




Transmission



  • Poor hygiene/sanitation

  • Fecal-oral transmission

  • Bedding

  • Clothing

  • Fingers after scratching

  • Slides also mention ventilation spread through inhaled eggs.




Location



  • Eggs hatch in duodenum

  • Mature in colon




Classic feature



Adult females emerge from the anus at night and deposit eggs around the perianal region.


The slides state that 5,000–16,000 eggs can be found perianally.



Diagnosis ⭐



Tape test


Tape is placed over the perianal region and examined microscopically.



Treatment



  • Antihelminthics

  • Improved hygiene






⭐ Must memorize:



Pinworm → nighttime itching/perianal eggs → tape test





11. Strongyloides stercoralis — Threadworm




Transmission



The skin comes into contact with fecally contaminated soil containing juveniles.



Life cycle



Juveniles:

Skin → blood → lungs → pharynx → intestinal mucosa


They develop into pathogenic females.


The slides emphasize reinfection through eggs → larvae → reinfection.



Diagnosis



Find a rhabditiform larva in feces.







12. 🦠 Protozoans



The review divides them into several groups.



Flagellates



Use flagella to move.


Examples:


  • Giardia lamblia

  • Trichomonas vaginalis

  • Trypanosoma brucei




Amoebas



Use pseudopods to move.


Examples:


  • Entamoeba histolytica

  • Entamoeba coli




Ciliates



Use cilia.


Example:


  • Balantidium coli




Sporozoans



Move by gliding.


Examples:


  • Plasmodium spp.

  • Toxoplasma gondii



The review also emphasizes the life stages cyst and trophozoite.



⭐ Movement memory:



Flagella → Flagellates


Pseudopods → Amoeba


Cilia → Ciliates


Gliding → Sporozoans





13. Giardia lamblia




Disease



Giardiasis



Location



Duodenum



Trophozoite



  • Heart-shaped

  • Active stage




Cyst



  • Dormant stage

  • Multinucleate; slide emphasizes two nuclei




Transmission



Fecally contaminated food or water



Symptoms



Mild cases may have none.


More advanced:


  • Diarrhea

  • Abdominal pain




Diagnosis



Trophozoites or cysts in stool







14. Trichomonas vaginalis




Disease



Trichomoniasis


Causes vulvovaginitis.



Location



Primarily:


  • Vulva

  • Vagina

  • Cervix



Can occur in males as well.



Transmission



STD



Diagnosis



Microscopic examination of discharge, with parabasal flagella noted in the slide.







15. Trypanosoma brucei



Three organisms are listed:



T. brucei brucei



  • Nonpathogenic




T. brucei gambiense



  • African sleeping sickness

  • West Africa

  • Mild/chronic

  • Can last years




T. brucei rhodesiense



  • African sleeping sickness

  • East Africa

  • Acute

  • Slide states death can occur within a year




Transmission



Tsetse fly



Important immune mechanism



The organism changes its surface antigens rapidly, making the immune response ineffective.



Diagnosis



Trypomastigotes can be found in:


  • Blood

  • CSF

  • Chancre at bite site



ELISA is also used.


The organism enters lymph, spreads through blood and can reach the heart, brain, and nervous system.



⭐ Must know:



Gambiense = West = chronic


Rhodesiense = East = acute


The images on page 18 show blood-smear examples of T. gambiense and T. rhodesiense.





16. Entamoeba histolytica




Disease



Amoebiasis / dysentery



Transmission



Fecal-oral


Through contaminated:


  • Food

  • Water



Associated with poor sanitation.



What does it feed on?



  • Red blood cells

  • Bacteria




Diagnosis



  • Trophozoites or cysts in stool

  • Blood and mucus in feces clinically




Cysts



Have 4 nuclei and are sturdy enough to survive stomach acid.


Page 19 emphasizes E. histolytica versus E. coli.





17. Plasmodium — Malaria




Disease



Malaria



Important organs/cells



  • Liver

  • Red blood cells




Transmission



Female Anopheles mosquito



Four species listed



  • P. vivax

  • P. malariae

  • P. falciparum

  • P. ovale




Effects



  • Fever

  • Anemia

  • Effects on spleen and liver




Diagnosis



Clinical findings plus identification of the parasite in blood smears.







18. Toxoplasma gondii




Disease



Toxoplasmosis



Cats



Cats are an important part of the life cycle.



Transmission highlighted



Raw meat/raw food



Symptoms



  • None to mild in many cases

  • Can be fatal in immunosuppressed individuals




Pregnancy



The slides specifically emphasize pregnancy because the organism can cross the placental barrier and cause encephalitis.



Diagnosis ⭐



Not microscopic


The slide says all stages are intracellular, so diagnosis is by testing for antibodies.


The slide’s image shows characteristic bow-shaped cells.







19. 🍄 Fungi



The important fungi listed are:



Yeasts [Y]



  • Candida albicans

  • Saccharomyces cerevisiae




Molds [M]



  • Rhizopus

  • Aspergillus

  • Penicillium






20. Aspergillus




Appearance



Slides describe:


  • Green, yellow, or brown granular colonies

  • White edge

  • Septate hyphae



A. niger produces black colonies.



Disease



Aspergillosis


The slides describe A. fumigatus as a cause.



Important concept



Aspergillus is ubiquitous, meaning it is found everywhere.


Infections are particularly associated with people who are immunocompromised or have other comorbidities.


Can cause:


  • Severe necrotizing pneumonia

  • Allergic aspergillosis



Page 24 visually shows characteristic Aspergillus structures and colonies.







21. Penicillium




Appearance



  • Green

  • Powdery

  • Radially furrowed colonies

  • White “apron”




Important facts



  • Penicillin associated with Alexander Fleming

  • Common contaminant

  • More than 200 species mentioned on the slide

  • P. marneffei can cause opportunistic infections in immunocompromised people

  • Other species cause spoilage




Commercial importance



Used in cheese fermentation, including:


  • Roquefort

  • Camembert

  • Brie



Also described as an ascomycete.


Page 24 contains several microscopic and culture images of Aspergillus and Penicillium.





22. 🔴 Acid-Fast Stain



This is extremely important.



Why use it?



For organisms with a waxy cell wall containing mycolic acid.


Mycolic acid prevents decolorization by acid-alcohol.



Major organisms



Mycobacterium


  • M. leprae

  • M. tuberculosis



Nocardia


  • N. brasiliensis

  • N. asteroides




Mycobacterium tuberculosis



The slides specifically mention a sputum specimen.



Stains



Primary stain: Carbolfuchsin = RED


Secondary/counterstain: Methylene blue



Acid-fast positive



Retains the red primary stain.



Acid-fast negative



Takes the counterstain.





Other organisms



The review also notes that some coccidian parasites have acid-fast oocysts, including:


  • Cryptosporidium

  • Isospora








23. 🟢 Capsule Stain




Capsule



A gelatinous covering around some cells that generally does not accept most dyes.



Glycocalyx



External to the cell wall and can consist of:


  • Mucoid polysaccharides

  • Polypeptides



Two forms:


Capsule


  • Organized

  • Firmly attached



Slime layer


  • Unorganized

  • Loosely attached




Why capsules matter



They:


  • Contribute to virulence

  • Help prevent phagocytosis

  • Help form biofilms




Capsule staining



Uses a background stain such as:


  • Nigrosin

  • Congo red



The background becomes stained while the capsule remains unstained, producing a halo around the cell.



Organisms emphasized



  • Bacillus anthracis — anthrax

  • Streptococcus pneumoniae — pneumonia

  • Klebsiella pneumoniae — pneumonia








24. 🟢 Endospore Stain




Endospores



Dormant, resistant structures produced inside certain bacterial cells.


They resist:


  • Heat

  • Chemicals

  • Poor environmental conditions

  • Desiccation

  • Radiation



The slides describe a tough keratin covering.



Schaeffer-Fulton method



Primary stain → Malachite green


Uses steam/heat.


Decolorizer → Water


Counterstain → Safranin



Appearance



Endospore = green


Vegetative cell = red/pink





Spore positions



Terminal → at the end


Central → middle; spherical/round


Subterminal → near the end; elliptical/oval



Sporulation



Formation of an endospore when nutrients are depleted.



Germination



Endospore returns to the vegetative state.



Organisms listed



  • Bacillus anthracis → anthrax

  • Clostridium tetani → tetanus

  • Clostridium botulinum → botulism

  • C. perfringens → gas gangrene

  • C. difficile → pseudomembranous colitis








25. 🧫 Streak Plate Technique



The last page covers bacterial isolation.



Simple zigzag



Used for samples with LOW density.



Quadrant streak plate



Used for samples with HIGH density.



Purpose



Used to isolate and separate mixed cultures.


The technique progressively decreases cell density so individual colonies can develop from individual cells.



CFU



CFU = Colony Forming Unit



Lab exercise



Students transfer:


  • Serratia

  • E. coli



to a plate.



Common errors



❌ Don’t break the agar by applying too much pressure.


❌ Don’t heat/sterilize the loop between each pass.