Microbiology · Chapter 2

Microscopy, Staining & the Microbiology Lab

Every microbe from Chapter 1 shares one problem. None of them can be seen with the naked eye. Yet microbiology has to name the germ in a wound, pick the right drug, and prove that a sterilizer worked. All of that starts with a way to make the invisible visible. This chapter is about that toolkit. You will meet the units used to measure things a thousand times smaller than a grain of salt. You will meet the microscope that enlarges them, the dyes that give them color, and the lab steps that turn a swab into a culture-and-sensitivity report. After this chapter, a lab result stops being a mystery. It starts being a set of instructions.

By the end of this chapter you can

State the metric units used to measure microbes. Place bacteria, viruses, and human cells on the size scale.
Explain why the unaided eye cannot resolve a bacterium.
Name the major parts of a compound light microscope and say what each one does.
Tell magnification apart from resolution, and explain the job of the oil-immersion lens.
Explain why cells must be stained, and compare simple, differential, and negative stains.
Walk through the four steps of the Gram stain and explain what makes a cell Gram-positive or Gram-negative.
Trace a specimen from collection through culture, colony, identification, and sensitivity testing.
List what good specimen collection needs, and read a culture-and-sensitivity report.

Section 1Why microbes are invisible

The relative sizes of microorganisms from viruses to protozoa, plotted against a metric scale — **Reference.** The relative sizes of microbes on a metric scale — viruses sit in the nanometer range, most bacteria near one micrometer, and protozoa are larger still. OpenStax Microbiology, CC BY 4.0 (via Wikimedia Commons)

The metric ladder

Microbiology runs on the metric system. The whole subject gets easier once the units feel normal. Start with the meter, then divide by a thousand at each step. A millimeter (mm) is one thousandth of a meter. A micrometer (µm) is one thousandth of a millimeter. Some people call it a micron. A nanometer (nm) is one thousandth of a micrometer. So each rung down is 1000 times smaller than the rung above it. You already live at the millimeter scale. The smallest lines on a ruler sit about a millimeter apart. Microbes live one or two rungs below that. Your eye was never built for that world.

How big is a microbe, really?

A red blood cell makes a handy yardstick. It is about 7 µm across. Most bacteria are smaller than that. A typical rod is about 0.5 µm wide and 2 to 3 µm long. A round coccus is about 1 µm. Yeasts and protozoa are eukaryotes, so they run bigger. They range from a few to tens of micrometers. Viruses sit in a whole other league. We measure them in nanometers. Most fall between 20 and 300 nm. Dozens of viruses could line up across one bacterium. For the smallest ones that count is closer to a hundred. This is why early microscopists never saw a virus. It took the electron microscope to bring them into view.

Unit	Size	What lives here
Millimeter (mm)	1/1000 of a meter	The smallest thing the naked eye resolves (about 0.1 mm)
Micrometer (µm)	1/1000 of a mm	Human cells (~10–100 µm), red blood cells (~7 µm), bacteria (~1–3 µm)
Nanometer (nm)	1/1000 of a µm	Viruses (~20–300 nm), large molecules, ribosomes

Why your eye can't see them

Your eye can tell two points apart only if they sit about 0.1 mm apart. That is 100 µm. Any closer and they blur into one dot. A bacterium is about 1 µm wide. That is roughly a hundred times below the limit. So it cannot register at all. Better eyesight will not help, and neither will brighter light. This is a hard physical limit, and it has a name: resolution. Beating it is the whole job of the microscope in the next section.

Scale explains why cultures take time. You cannot see one bacterium. You can see the pile of a billion of them that grows overnight into a visible colony. That jump from invisible to visible is why a wound culture does not come back in five minutes. The lab is waiting for the organism to multiply into something the eye, and the tests, can work with.

Interactive Slide the zoom from millimeters to nanometers to see why a virus, a bacterium, and a human cell are all invisible. Open full ↑

Key points

Metric ladder: 1 mm = 1000 µm, and 1 µm = 1000 nm. Each step is 1000× smaller.
Most bacteria are ~1–3 µm. Viruses are ~20–300 nm. A red blood cell (~7 µm) is a handy yardstick.
The naked eye resolves only to about 0.1 mm. That is roughly 100× too coarse to see a bacterium.
One microbe is invisible, so the lab grows it into a visible colony before it can work with it.

Section 2The light microscope

A labeled compound brightfield light microscope showing its major parts — **Reference.** A compound brightfield light microscope — light passes up from the illuminator through the condenser, specimen, objective, and ocular to your eye. OpenStax Microbiology, CC BY 4.0 (via Wikimedia Commons)

The parts and what they do

Every clinical lab leans on the compound light microscope. It is called compound because it uses two lens systems in a row to build up its magnifying power. Light starts at the illuminator in the base and travels upward. The condenser gathers that light and focuses it on the specimen. The iris diaphragm sets how much light gets through, and that controls contrast. The specimen rests on the stage. Above it sit the objective lenses, held on a turning nosepiece. They do the first round of magnifying. The ocular, or eyepiece, does the second round. The coarse and fine focus knobs raise and lower the stage until the image is sharp.

Magnification: stacking two lenses

Total magnification is the ocular times the objective. Oculars are almost always 10×. Objectives come in a standard set: scanning (4×), low power (10×), high-dry (40×), and oil immersion (100×). So a 10× ocular with the oil lens gives 10 × 100, or 1000× in all. That is enough to read bacterial shape and arrangement. Find your target on a low-power objective first. Center it. Then turn up to a higher power.

Objective	Objective power	With 10× ocular	Typical use
Scanning	4×	40×	Find the specimen, survey the slide
Low power	10×	100×	Larger cells, general orientation
High-dry	40×	400×	Fungi, cells, parasites
Oil immersion	100×	1000×	Bacteria — Gram stains, cell arrangement

Resolution is what really matters

Beginners chase magnification. The thing that decides whether an image is useful is resolution. Resolution is the smallest gap between two points that still lets you see them as separate. Blow up a blurry image and you only get a bigger blur. Lab staff call that empty magnification. The wavelength of visible light caps the resolution of a light microscope near 0.2 µm. You cannot split two points closer than about half a wavelength. That 0.2 µm limit is fine for bacteria. It is not fine for viruses, which is why viruses need an electron microscope. To get the best out of the 100× lens, add a drop of immersion oil between the slide and the lens. Oil bends light the same way glass does, because its refractive index matches. So light rays stop scattering away as they leave the slide. More of them reach the lens, and the image gets sharper.

Never skip the oil, and never drag a dry lens through it. The 100× objective is built to work in oil. Used dry, the image is dim and unresolved. And the 40× high-dry lens should never swing into a pool of oil, because it is not sealed for it. It is a small habit, but it is the difference between reading a Gram stain and squinting at mush.

Interactive Tap the microscope parts, then dial the objective and the oil to watch magnification and resolution pull apart Open full ↑

Key points

The compound light microscope uses two lenses. Light travels illuminator → condenser → specimen → objective → ocular.
Total magnification = ocular × objective. The 10× ocular with the 100× oil lens gives 1000×.
Resolution (~0.2 µm for light), not magnification, sets how much real detail you see.
Immersion oil matches the refractive index of glass. It catches more light rays and sharpens the 100× image.

Section 3Why we stain

Comparison of positive staining, which colors the cells, and negative staining, which colors the background — **Reference.** Positive staining colors the cells against a clear field; negative staining colors the background and leaves the cells clear. OpenStax Microbiology, CC BY 4.0 (via Wikimedia Commons)

The contrast problem

Living cells are mostly water and almost clear. Under a plain light microscope they nearly vanish against the bright background. There is not enough contrast to show shape or structure. Staining fixes this by coating the cell in colored dye. First you make a smear. Spread a thin film of the sample on a slide and let it air-dry. Then pass it through a flame, or warm it, to heat-fix it. Fixing does three jobs. It kills most of the organisms. It glues them to the glass so they do not rinse away. It holds their shape. But heat-fixing is not sterilization. It kills most vegetative bacteria, while mycobacteria and bacterial endospores can live through it. Most bacterial dyes are basic dyes, which carry a positive charge. Methylene blue, crystal violet, and safranin are all basic. The bacterial surface carries a negative charge, so it pulls them in.

Simple stains

A simple stain uses one dye to add color and contrast. It shows a cell's size, shape, and arrangement. You can tell a round coccus from a rod-shaped bacillus. You can see clusters, chains, or pairs. It tells you nothing more than that. It is fast, and it is the right tool when all you need is proof that organisms are there.

Differential and special stains

A differential stain uses more than one reagent. It sorts bacteria into groups by how they react. Two of them matter most. The Gram stain fills the next section. The acid-fast stain picks out the waxy-walled organisms, such as Mycobacterium tuberculosis, that a Gram stain misses. Special stains go after one structure. There are capsule stains, endospore stains, and flagella stains. Many of these use negative staining. An acidic dye such as nigrosin, or a fine suspension like India ink, colors the background instead of the cell. The cell stays unstained and glows against the dark field. Negative staining skips heat-fixing. So it does not warp delicate cells or shrink a capsule. That makes it the first choice for capsules and for fragile organisms.

Stain type	Reagents	What it tells you	Example
Simple	One dye	Size, shape, arrangement	Methylene blue
Differential	Two or more reagents	Sorts cells into groups	Gram stain, acid-fast stain
Special	Structure-specific	Reveals one feature	Capsule, endospore, flagella
Negative	Acidic dye (stains background)	Shape without distortion; capsules	India ink, nigrosin

A stain is a snapshot, not a sterilizer. Heat-fixing kills most vegetative bacteria and glues them to the slide, but it does not sterilize. Mycobacteria and endospores can stay alive, so a fixed acid-fast smear is still handled as infectious material in a biosafety cabinet. That is why the lab keeps the plate growing separately. The Gram stain gives a fast first read, while the living culture goes on to identification and sensitivity testing.

Interactive Put one smear through six stains and watch how much each one can tell you Open full ↑

Key points

Cells are nearly clear, so staining adds the contrast you need to see shape and structure.
A smear is air-dried and heat-fixed. That kills most organisms, fixes them to the slide, and holds their shape. It does not sterilize.
Simple stains use one dye and show shape. Differential stains use several reagents and sort cells into groups.
Negative staining colors the background and skips the heat, so it suits capsules and fragile cells.

Section 4The Gram stain

The four-step Gram stain procedure, showing how Gram-positive and Gram-negative cells respond at each step — **Reference.** The Gram-stain procedure — crystal violet, iodine mordant, alcohol decolorizer, and safranin counterstain leave Gram-positive cells purple and Gram-negative cells pink. OpenStax Microbiology, CC BY 4.0 (via Wikimedia Commons)

What makes a cell Gram-positive or Gram-negative

Hans Christian Gram devised this stain in 1884. It is still the single most important stain in clinical microbiology. It works because of a real difference in the bacterial cell wall. Gram-positive bacteria have a thick outer layer of peptidoglycan. Peptidoglycan is a mesh-like polymer, and the thick layer acts like a sponge. Gram-negative bacteria have only a thin peptidoglycan layer. Wrapped around it is an extra outer membrane. The four reagents play off that difference. The thick wall traps the first dye. The thin wall lets it wash out.

The four steps

The order never changes, and every step has a job. First comes the primary stain, crystal violet. It floods every cell purple. Second comes Gram's iodine, which acts as a mordant. It binds the crystal violet into a large complex, too big to slip out of a thick wall easily. Third comes the decolorizer, alcohol or an alcohol-acetone mix. This is the step that truly splits the two groups. Alcohol dries and tightens the thick Gram-positive wall, so the dye complex stays locked inside. Alcohol also dissolves the Gram-negative outer membrane. The dye then washes straight out, and those cells turn colorless. Fourth comes the counterstain, pink safranin. It re-colors the now-colorless Gram-negative cells. The Gram-positive cells are already full of purple, so safranin does not change them. The end result: Gram-positive = purple, Gram-negative = pink or red.

Step	Reagent	Role	Gram-positive	Gram-negative
1	Crystal violet	Primary stain	Purple	Purple
2	Gram's iodine	Mordant (traps dye)	Purple	Purple
3	Alcohol / acetone	Decolorizer (the key step)	Stays purple	Becomes colorless
4	Safranin	Counterstain	Stays purple	Turns pink/red

Reading the result

A Gram stain can be read in under an hour. That is long before a culture matures. So it gives the first real clue for choosing empiric antibiotics, while the lab waits 24 to 48 hours for growth. The report pairs the Gram reaction with the shape and the arrangement, and each pairing narrows the field. Gram-positive cocci in clusters point to Staphylococcus. Gram-positive cocci in chains point to Streptococcus. Gram-negative rods suggest gut organisms such as E. coli. Gram-negative diplococci raise Neisseria. That wall difference matters for treatment too. The Gram-negative outer membrane blocks some antibiotics, and it carries endotoxin, which Chapter 10 covers. That is one reason Gram-negative bloodstream infections can turn so dangerous.

The decolorizer is where Gram stains go wrong. Leave the alcohol on too long and even a Gram-positive cell gives up its purple and reads as a false Gram-negative. Rinse too briefly and Gram-negatives hold onto the dye and read as a false Gram-positive. When a stain result does not fit the clinical picture, too much or too little decolorizing is the usual culprit. That is another reason the living culture, not the stain alone, gets the final word.

Interactive Run the four reagents yourself, set the decolorizer timer, and watch both cells take or lose their color Open full ↑

Key points

The Gram stain sorts bacteria by cell wall: thick peptidoglycan for Gram-positive, thin peptidoglycan plus an outer membrane for Gram-negative.
Four steps: crystal violet → iodine mordant → alcohol decolorizer → safranin counterstain.
Decolorizing is the make-or-break step that tells the two groups apart.
Result: Gram-positive = purple, Gram-negative = pink. Read in under an hour, it guides the early antibiotic choice.

Section 5From specimen to answer

A Kirby-Bauer disk-diffusion plate showing zones of inhibition around antibiotic disks — **Reference.** Disk-diffusion (Kirby-Bauer) sensitivity testing — clear zones of inhibition around antibiotic disks show which drugs stop the organism. OpenStax Microbiology, CC BY 4.0 (via Wikimedia Commons)

Collecting and culturing the specimen

A lab result is only as good as the specimen behind it. The rules for collection are simple and strict. Sample the right site, meaning the infection itself and not the skin around it. Collect before antibiotics start, whenever that is possible. Use the correct container and transport medium. Label it accurately at the bedside. Back in the lab, the sample is spread onto a culture medium. That is usually a nutrient agar plate, a solid jelly, or a liquid broth. Some media are picked on purpose. Selective media hold back unwanted organisms. Differential media, such as blood agar or MacConkey, make different species look different. That gives the lab an early hint about what is growing.

The colony: an invisible bug made visible

To isolate one organism, the lab uses a streak plate. The sample is dragged thinner and thinner across the agar, until single cells sit far apart. Leave one of those cells overnight in a warm incubator. It divides again and again into a visible mound of millions of identical clones. That mound is a colony. Every cell in it came from one parent, so a colony is a pure sample to work from. Its morphology is a first clue to identity. Morphology covers size, shape, color, and texture. It also covers whether the organism breaks down the blood in blood agar, which is called hemolysis. From there, biochemical panels, automated analyzers, or fast MALDI-TOF mass spectrometry pin down the species.

Sensitivity testing and the antibiogram

The organism's name is only half the answer. Someone still needs to know which antibiotics will kill it. That is sensitivity testing, also called susceptibility testing. The classic method is Kirby-Bauer disk diffusion. Small paper disks, each soaked with a different antibiotic, sit on a lawn of the organism. Where a drug works, it spreads outward and clears a zone of inhibition. A larger zone usually means the organism is more susceptible to that drug. Each zone is measured in millimeters and matched against standard breakpoints. You do not size it up against the disk next door. Other methods measure the minimum inhibitory concentration (MIC), the lowest drug level that stops growth. Broth microdilution and a gradient Etest strip both report it. Results come back as S, I, or R, which stand for susceptible, intermediate, and resistant. This is the report that lets a team de-escalate. Start broad while you wait, then narrow to the most targeted drug that works once the sensitivities return.

Stage	Typical timing	What you learn
Gram stain	< 1 hour	Gram reaction, shape, arrangement — guides empiric therapy
Colony growth	18–48 hours	Preliminary identity from colony morphology
Identification & sensitivity	48–72 hours	Species and the S/I/R susceptibility report

Cultures before antibiotics, always. Once a dose is on board, the organism may stop growing in the bottle, and the culture can come back falsely negative. That robs the team of the identity and the sensitivities that would have targeted therapy. Blood cultures in particular should be drawn before the first antibiotic dose. Getting the culture first is one of the highest-value steps in a suspected sepsis workup.

Interactive Drive the whole lab pathway: collect, plate, streak, identify, and measure the sensitivity zones Open full ↑

Key points

Good results start with good collection: right site, before antibiotics, correct container, accurate label.
A streak plate isolates single cells. Each one grows into a pure colony whose morphology hints at identity.
Sensitivity testing uses Kirby-Bauer zones or an MIC, and reports each drug as S/I/R.
The culture-and-sensitivity report lets a team de-escalate from broad empiric therapy to one targeted drug.

The chapter in one breath

Microbes are measured in micrometers (bacteria, ~1–3 µm) and nanometers (viruses, ~20–300 nm). The naked eye stops at ~0.1 mm, so one microbe is invisible.
The compound light microscope stacks two lenses. Total magnification is ocular × objective, and it reaches 1000× with the oil lens.
Resolution, not magnification, sets useful detail. Light microscopy bottoms out near 0.2 µm, and immersion oil sharpens the 100× image.
Cells are nearly clear, so you make a heat-fixed smear and stain it. Simple stains show shape, differential stains sort cells into groups, and negative stains color the background.
The Gram stain uses four steps: crystal violet, iodine, alcohol, safranin. They play off the cell wall, so Gram-positive = purple and Gram-negative = pink. Decolorizing is the key step.
A specimen becomes an answer through culture, a pure colony, species identification, and sensitivity testing reported as S/I/R.
Good results start at the front end. Collect the specimen properly, draw cultures before antibiotics, then use the culture-and-sensitivity report to push for the right drug.

ReferenceWords worth knowing

A quick reference for the terms in this chapter. Skim it now, or circle back whenever a word trips you up.

Acid-fast stain: A differential stain that finds organisms with waxy cell walls, such as Mycobacterium tuberculosis, which a Gram stain does not show well.
Agar: A solid gel made from seaweed, used as the base of culture plates that microbes grow on.
Antibiogram: A table of which antibiotics work against an organism, with each drug marked susceptible, intermediate, or resistant. Hospitals also publish a combined antibiogram that pools the results from all the isolates they see over a year.
Colony: A visible mound of millions of identical cells, all descended from one original cell.
Compound light microscope: A microscope that uses two lens systems in a row, the ocular and the objective, to magnify a specimen.
Counterstain: The second-color dye in a differential stain, safranin in the Gram stain, that re-colors the cells the decolorizer has left colorless.
Culture: The growth of microbes on or in a nutrient medium in the lab.
Decolorizer: The alcohol or acetone step of the Gram stain. It pulls dye out of Gram-negative cells, while Gram-positive cells hold onto it.
Differential stain: A staining method that uses two or more reagents to tell groups of bacteria apart. The Gram stain is the classic one.
Fixation: Treating a smear with heat or a chemical to kill the organisms, stick them to the slide, and hold their shape before staining.
Gram stain: The key differential stain. It sorts bacteria into Gram-positive (purple) and Gram-negative (pink) by cell-wall structure.
Magnification: How much larger an image looks than the real object. It is the ocular power times the objective power.
Micrometer (µm): One thousandth of a millimeter. It is the unit used for most bacteria and for eukaryotic cells.
Minimum inhibitory concentration (MIC): The lowest level of an antibiotic that stops an organism from growing where you can see it.
Mordant: A substance, Gram's iodine here, that binds a dye more firmly to the cell and traps it in the wall.
Negative staining: Staining the background with an acidic dye so the cells stay clear. It skips heat, so it keeps capsules and fragile cells intact.
Objective lens: The lens nearest the specimen (4×, 10×, 40×, 100×) that does the first round of magnifying.
Oil immersion: Putting immersion oil between the slide and the 100× lens. The oil's refractive index matches glass, which improves resolution.
Peptidoglycan: The mesh-like polymer of the bacterial cell wall. It is thick in Gram-positive cells and thin in Gram-negative cells.
Resolution: The smallest gap between two points at which you can still see them as separate. It is about 0.2 µm for a light microscope.
Simple stain: A stain that uses one dye to show a cell's size, shape, and arrangement.
Zone of inhibition: The clear ring around an antibiotic disk in disk-diffusion testing, where the drug has stopped bacteria from growing.

Check yourself

Interactive Read a fresh Gram-stained field, call the reaction, shape, and arrangement, and build the report line yourself Open full ↑

1. Put these in order from largest to smallest and give an approximate size for each: a virus, a bacterium, a red blood cell.

Red blood cell (~7 µm) > bacterium (~1–3 µm) > virus (~20–300 nm). Bacteria are measured in micrometers and viruses in nanometers. Dozens of viruses could line up across one bacterium.

2. Why can't the naked eye see one bacterium, and how does the lab get around this without a microscope on every plate?

The eye resolves only to about 0.1 mm (100 µm), roughly 100× too coarse for a 1-µm bacterium. The lab grows that invisible single cell into a visible colony of billions of clones, which the eye and the tests can work with.

3. A microscope is set to 400× total magnification. Which ocular and objective are in use?

A 10× ocular with the 40× high-dry objective (10 × 40 = 400×). Total magnification is always the ocular power times the objective power.

4. Explain the difference between magnification and resolution, and why "empty magnification" is a problem.

Magnification makes the image bigger. Resolution is the smallest gap at which two points still look separate. Magnifying past the resolution limit (~0.2 µm for light) only enlarges a blur. That is empty magnification: more size, no new detail.

5. What does immersion oil do, and why is it used only with the 100× objective?

Oil has the same refractive index as glass, so it keeps light rays from scattering away and funnels more of them into the lens. That improves resolution. The 100× lens is built for oil. The 40× high-dry lens is not sealed for it and should not be dragged through oil.

6. Why must bacteria be stained to be studied, and what three things does heat-fixing accomplish?

Cells are nearly clear and have no contrast, so dye makes them visible. Heat-fixing kills most vegetative organisms, sticks them to the slide, and holds their shape. It is not sterilization, so mycobacteria and endospores may survive, and fixed smears are still handled as infectious.

7. Contrast a simple stain with a differential stain, and give an example of each.

A simple stain uses one dye and shows only size, shape, and arrangement, as methylene blue does. A differential stain uses several reagents to sort cells into groups, as the Gram and acid-fast stains do. Differential stains carry grouping information that a simple stain cannot.

8. When would you choose a negative stain over a positive (simple) stain?

When you need to keep delicate structures intact, classically to show a capsule or to view fragile cells. Negative staining colors the background and skips heat-fixing, so nothing gets warped. India ink and nigrosin are the usual choices.

9. List the four Gram-stain reagents in order and state the result for a Gram-negative cell after each.

Crystal violet (purple) → iodine mordant (purple) → alcohol decolorizer (colorless) → safranin counterstain (pink or red). Gram-negative cells lose the crystal violet at decolorizing, because alcohol dissolves their outer membrane and thin wall. Then they take up the pink counterstain.

10. Which step of the Gram stain most often produces a wrong result, and how does the error happen in each direction?

Decolorizing is the critical step, and the easiest one to get wrong. Too much washes purple out of a true Gram-positive, so it reads falsely Gram-negative. Too little leaves dye in a Gram-negative, so it reads falsely Gram-positive.

11. A wound Gram stain reports "Gram-positive cocci in clusters." What organism does that suggest, and why is this result useful before the culture is finished?

Gram-positive cocci in clusters classically suggest Staphylococcus. The Gram stain is read in under an hour, so it guides the empiric antibiotic choice during the 24 to 48 hours the culture needs to grow.

12. A patient with suspected sepsis is about to receive antibiotics. Why should blood cultures be drawn first, and what do disk-diffusion zones of inhibition tell the team?

Antibiotics can hold back growth in the bottle and cause a falsely negative culture, so cultures must be drawn before the first dose. In disk diffusion, a clear zone around a drug disk means that antibiotic stops the organism. A larger zone usually reflects greater susceptibility, which lets the team de-escalate to targeted therapy.

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