Bacterial Diversity & Classification
There are more kinds of bacteria than kinds of anything else alive. Only a small share of them ever make a person sick. Those few are the ones worth knowing well. The way to keep them straight is not a long list. It is a short set of features that microbiologists use to sort bacteria into groups. Shape, staining, oxygen needs, and a few odd traits place almost any germ into a box. The box then tells you how it spreads, what illness it causes, and which drugs will work. This chapter walks through those sorting features one at a time. It hangs the most important germs on them. By the end, a culture report or a Gram stain result reads like a sentence you already know.
By the end of this chapter you can
- Name the three basic bacterial shapes and the common arrangements, and read the clues hidden in a genus name.
- Explain what the Gram stain shows, and why gram-positive and gram-negative walls react in different ways.
- Identify the major gram-positive germs — Staphylococcus, Streptococcus, and Clostridium or Clostridioides — and the diseases they cause.
- Identify the major gram-negative germs — E. coli, Salmonella, and Pseudomonas — and explain why endotoxin makes them dangerous.
- Describe the special bacteria — mycobacteria, spirochetes, and mycoplasmas — that break the usual staining and cell wall rules.
- Sort bacteria by their oxygen needs, and explain how that changes where infections form and how samples are collected.
- List the other axes used to describe bacteria: temperature, pH, salt, and life inside host cells.
- Tie each sorting feature to a real step in testing, cleaning, or treatment.
Section 1Shapes & arrangements
Three basic shapes
Under the microscope, almost every bacterium takes one of three shapes. The lab reports the shape first, because it narrows the field so fast. A coccus (plural cocci) is round, like a tiny ball. A bacillus (plural bacilli) is a rod. Picture a short stick or a grain of rice. A spirillum (plural spirilla) is curved or coiled. Picture a corkscrew, or a comma. Two in-between forms are worth knowing too. A coccobacillus is a short, plump rod that is easy to mistake for a coccus. A vibrio is a short curve shaped like a comma. That shape is what gives Vibrio cholerae its name.
Shape is more than a label. It is built into the name of almost every germ in this book. Once you can hear it, the words begin teaching themselves. Staphylococcus and Streptococcus are spheres. Bacillus and Lactobacillus are rods. Spirochetes are coils. Reading the shape out of a genus name is a habit worth building on day one.
Arrangements: how the cells hang together
When a bacterium splits in two, the daughter cells may drift apart. They may also stay stuck together. The pattern they make is a second clue, and it helps most with cocci. The prefixes make sense once you learn them. Diplo- means the cells sit in pairs. Those are diplococci, as in Neisseria. Strepto- means they form chains, like beads on a string. Those are streptococci. Staphylo- means they pile up in loose clumps shaped like a bunch of grapes. Those are staphylococci. Cocci can also form flat squares of four, called tetrads. They can form cube-shaped packets of eight, called sarcinae. Rods behave a little differently. They more often line up end to end in short chains. They can also lie side by side like a fence. That side-by-side pattern is a palisade, and it is typical of Corynebacterium.
| What you see | Name | Classic organism |
|---|---|---|
| Round, in grape-like clusters | Staphylococci | Staphylococcus aureus |
| Round, in chains | Streptococci | Streptococcus pyogenes |
| Round, in pairs | Diplococci | Neisseria, S. pneumoniae |
| Round, in flat squares of four | Tetrads | Micrococcus luteus |
| Rod-shaped | Bacillus | E. coli, Clostridium |
| Rods side by side, like a fence | Palisade | Corynebacterium diphtheriae |
| Comma or curved rod | Vibrio | Vibrio cholerae |
| Spiral or corkscrew | Spirochete | Treponema pallidum |
- The three basic shapes are coccus (sphere), bacillus (rod), and spirillum (spiral). Vibrios and coccobacilli are in-between forms.
- Arrangement prefixes add a second layer of clues, mostly for cocci: diplo- for pairs, strepto- for chains, staphylo- for clusters.
- Shape and arrangement are usually built into the genus name, so the name itself carries hints.
- The early Gram stain report gives shape, arrangement, and color. It guides the first drug choice before the final name arrives.
Section 2Gram-positive pathogens
What "gram-positive" means
Hans Christian Gram worked out the Gram stain in 1884. It sorts almost all bacteria into two great camps. The split turns on how the cell wall holds a purple dye. Gram-positive bacteria have a thick outer layer of peptidoglycan. Peptidoglycan is a mesh of sugars and short proteins, and it gives the wall its strength. The thick mesh traps the crystal violet dye, so the cells look deep purple. That wall also sits bare on the surface, with nothing wrapped over it. So gram-positive germs are the classic targets of the penicillin family. A penicillin wrecks peptidoglycan while the cell is building it. The three groups below cause a huge share of all gram-positive disease.
Staphylococcus — the cluster
Staphylococcus cells are gram-positive cocci in grape-like clusters. The species that matters most is Staphylococcus aureus. It is a remarkably flexible pathogen. Roughly a third of healthy people carry it in the nose and never know. Give it a break in the skin and the picture changes fast. It causes boils, abscesses, wound infections, and cellulitis, a spreading infection of the skin and the tissue under it. It causes bloodstream infections. It causes endocarditis, an infection of the heart valves. Its toxins cause one kind of food poisoning and toxic shock syndrome. MRSA is short for methicillin-resistant S. aureus. It is the drug-resistant form, and it is why so many hospital units use contact precautions. A related group is the coagulase-negative staphylococci, such as S. epidermidis. These live quietly on skin and are weak on their own. They still cause trouble by settling on catheters, artificial joints, and heart valves.
Streptococcus — the chain
Streptococcus cells are gram-positive cocci in chains. The lab sorts them two ways. One is how they break down blood on an agar plate. The other is the marker proteins on their surface. Group A strep is S. pyogenes. It causes strep throat, impetigo, cellulitis, and scarlet fever. It also causes necrotizing fasciitis, a fast infection that destroys tissue. Weeks later it can set off two immune problems. One is rheumatic fever, which can scar heart valves. The other is post-streptococcal glomerulonephritis, an injury to the filters of the kidney. Group B strep is S. agalactiae. It lives in the vagina of some pregnant people without causing symptoms. It is a leading cause of newborn sepsis and meningitis. That is why screening is done late in pregnancy, at about 36 to 37 weeks. Streptococcus pneumoniae grows as a pair, not a chain. It is a top cause of pneumonia caught outside the hospital. It also causes middle ear infection, called otitis media, and meningitis. It is the target of the pneumococcal vaccines.
Clostridium — the spore-former
Clostridium and its close relative Clostridioides are gram-positive rods. They have two dangerous habits. First, they are obligate anaerobes, a term Section 5 unpacks. Second, they form endospores. An endospore is a dormant, armored survival capsule. It shrugs off heat, drying, and alcohol. The roll call is easy to remember. Clostridioides difficile causes colitis after antibiotics. Clostridium tetani causes tetanus. Clostridium botulinum causes botulism. Clostridium perfringens causes gas gangrene and a form of food poisoning. Because the spores resist alcohol, an alcohol hand rub does not remove C. difficile. Hands must be washed with soap and water. Surfaces need a bleach cleaner, which does kill spores.
| Genus | Look | Key species & disease | What it changes |
|---|---|---|---|
| Staphylococcus | Cocci in clusters | S. aureus — skin, wound, blood; MRSA | Contact precautions for MRSA |
| Streptococcus | Cocci in chains | Group A (throat, necrotizing fasciitis); Group B (newborn sepsis); pneumoniae (pneumonia) | Screen for group B late in pregnancy; vaccinate against pneumococcus |
| Clostridium / Clostridioides | Rods, spore-forming, anaerobic | C. difficile, tetanus, botulism, gas gangrene | Soap and water, not alcohol, for C. diff |
- Gram-positive bacteria have thick peptidoglycan, stain purple, and are the classic targets of penicillins.
- Staphylococcus means cocci in clusters. S. aureus and MRSA cause skin, wound, and bloodstream infections.
- Streptococcus means cocci in chains. Group A brings throat infection, necrotizing fasciitis, and rheumatic fever. Group B brings newborn sepsis. S. pneumoniae brings pneumonia and meningitis.
- Clostridium and Clostridioides are anaerobic, spore-forming rods. C. diff spores resist alcohol, so wash with soap and water.
Section 3Gram-negative pathogens
What "gram-negative" means, and why endotoxin matters
Gram-negative bacteria carry only a thin layer of peptidoglycan. They lose the purple crystal violet during the wash step. They then take up the pink counterstain instead. What really sets them apart is a second membrane. It wraps around the outside of the peptidoglycan, and it is called the outer membrane. Its outer face is studded with lipopolysaccharide, or LPS. One part of LPS, called lipid A, is a strong endotoxin. An endotoxin is a poison built into the cell wall itself. When gram-negative cells die and break apart in the blood, lipid A spills out. The immune system then overreacts. Fever climbs, blood pressure drops, and at the far end septic shock follows. This is why a gram-negative bloodstream infection is an emergency. Many sepsis alerts trace back to these germs.
Escherichia coli — the everyday gut rod
Escherichia coli is a gram-negative rod. It lives in the colon of every healthy person and does no harm there. Out of place, it is the single most common cause of urinary tract infections. It is also a frequent cause of gram-negative sepsis and of meningitis in newborns. A few strains are true gut pathogens. Enterohemorrhagic E. coli, such as the strain O157:H7, makes Shiga toxin. It can cause bloody diarrhea and hemolytic uremic syndrome. That syndrome shreds red blood cells and is a cause of sudden kidney failure in children. E. coli is a facultative anaerobe. That is why it does well both in the low-oxygen gut and in the bladder.
Salmonella — the food-borne invader
Salmonella are gram-negative rods. They spread through food and water, and through contact with reptiles and poultry. Most infections are a short, self-limited gastroenteritis. That is the classic food poisoning from undercooked eggs or chicken. One type behaves very differently. Salmonella Typhi causes typhoid fever, an illness that spreads through the whole body with a long high fever. Salmonella carries a useful lesson. In plain infectious diarrhea in an otherwise healthy adult, fluids and hand washing do the work. Routine antibiotics do not.
Pseudomonas aeruginosa — the opportunist of wet places
Pseudomonas aeruginosa is a gram-negative rod and an obligate aerobe. It is one of the great opportunists of the hospital. An opportunist strikes only when defenses are already down. It lives in wet spots: sink drains, breathing equipment, standing water, even flower vases. It causes pneumonia in people on ventilators. It infects burns and long-standing wounds. It causes urinary infections tied to catheters. It settles into the lungs for years in cystic fibrosis. It makes a blue-green pigment and smells like grapes or corn tortillas. It is also intrinsically resistant to many antibiotics, which means the resistance is built in from the start. That makes it hard to treat and a common driver of multidrug-resistant outbreaks.
| Organism | Oxygen | Signature disease | Where it lurks |
|---|---|---|---|
| Escherichia coli | Facultative anaerobe | UTI, gram-negative sepsis; O157:H7 → HUS | Colon (normal flora) |
| Salmonella | Facultative anaerobe | Gastroenteritis; typhoid fever | Eggs, poultry, reptiles |
| Pseudomonas aeruginosa | Obligate aerobe | Ventilator pneumonia, burn and wound infection, cystic fibrosis lung | Water, moist equipment |
- Gram-negative bacteria stain pink and carry an outer membrane with LPS. Its lipid A is the endotoxin that can drive septic shock.
- E. coli is the top cause of UTIs and a common cause of gram-negative sepsis. Some strains, such as O157:H7, cause bloody diarrhea and HUS.
- Salmonella is food-borne. Most infections are self-limited gastroenteritis, but S. Typhi causes typhoid fever.
- Pseudomonas aeruginosa is a water-loving, drug-resistant opportunist. It is behind ventilator pneumonia, burn and wound infection, and cystic fibrosis lung disease.
Section 4Special bacteria
Mycobacteria — the waxy, acid-fast rods
A few important bacteria refuse to fit the Gram stain scheme. The mycobacteria matter most among them. Their cell wall is loaded with a waxy fat called mycolic acid. The wax repels ordinary water-based dyes. To see these cells you need an acid-fast stain. Heat and a strong dye push color into the wall. The color then holds on, even when acid-alcohol is used to wash it out. Mycobacterium tuberculosis causes tuberculosis, or TB. Its traits drive real practice. The waxy wall makes it slow to grow, so cultures take weeks. The same wax makes it tough out in the environment. TB travels through the air on tiny dried droplets called droplet nuclei. So it calls for airborne isolation: a negative-pressure room and a fitted N95 respirator, not a plain surgical mask. Its cousin Mycobacterium leprae causes leprosy, also called Hansen disease.
Spirochetes — the corkscrews
Spirochetes are long, thin, tightly coiled bacteria. They move with a corkscrew twist. The motor is a set of internal axial filaments, also called endoflagella. These cells are so slender that a routine Gram stain often misses them. Most will not grow on ordinary media either. So they are usually found by blood tests rather than by culture. Three are worth knowing. Treponema pallidum causes syphilis. Borrelia burgdorferi causes Lyme disease and spreads by tick bite. Leptospira causes leptospirosis, picked up from water holding animal urine. Because they hide so well, antibody tests and a careful history carry the diagnosis.
Mycoplasma — the wall-less minimalists
Mycoplasma are the smallest free-living bacteria. They are also unique in one way: they have no cell wall at all. With no rigid wall they have no set shape. The word for that is pleomorphic. No wall also means no peptidoglycan. A penicillin has nothing to grab, so beta-lactam drugs do not work on them. Beta-lactams are the penicillin and cephalosporin family. Mycoplasma pneumoniae causes "walking pneumonia." That is a mild, drawn-out lung infection, common in younger people. It is treated with a macrolide or a tetracycline instead of a penicillin. The lesson runs the opposite way from usual. A drug works by hitting a part the germ actually has. No wall means no target.
| Group | What breaks the rules | Key organism / disease | What follows from it |
|---|---|---|---|
| Mycobacteria | Waxy mycolic-acid wall; acid-fast | M. tuberculosis — TB | Airborne isolation; slow culture |
| Spirochetes | Thin coils; hard to stain or culture | T. pallidum (syphilis), B. burgdorferi (Lyme) | Found by blood antibody tests, not culture |
| Mycoplasma | No cell wall at all | M. pneumoniae — walking pneumonia | Penicillins fail; use a macrolide or tetracycline |
- Mycobacteria have a waxy mycolic-acid wall, are acid-fast, and grow slowly. M. tuberculosis needs airborne isolation.
- Spirochetes are thin corkscrews: Treponema, Borrelia, and Leptospira. They resist staining and culture, so antibody tests find them.
- Mycoplasma has no cell wall, so beta-lactam drugs fail. M. pneumoniae causes walking pneumonia.
- Each special group breaks one rule. Each break changes the stain, the isolation, or the drug.
Section 5Oxygen & other groupings
Sorting bacteria by their relationship to oxygen
Oxygen keeps your own cells alive. To bacteria it can be vital, pointless, or deadly. How a germ handles oxygen is a core sorting feature, with real consequences. An obligate aerobe must have oxygen to grow. Pseudomonas and Mycobacterium tuberculosis are examples. An obligate anaerobe is poisoned and killed by oxygen. It grows only where oxygen is missing, as Clostridium and Bacteroides do. A facultative anaerobe sits in the flexible middle. It prefers oxygen but can switch to fermentation without it. E. coli and Staphylococcus belong here, along with most germs that grow easily in the lab. A microaerophile needs oxygen, but only a little of it. Campylobacter and Helicobacter pylori are the examples. An aerotolerant anaerobe never uses oxygen, yet air does it no harm.
Why the oxygen group changes what gets done
This is not a dry detail. Anaerobes thrive exactly where oxygen cannot reach. Think of the depths of a puncture wound. Think of the center of an abscess, a walled-off pocket of pus. Think of crushed or dead tissue, the gut, and the pockets around the teeth. That is why deep, dirty, and bite wounds are anaerobe problems, and so is gas gangrene. Oxygen also changes how samples are handled. Air kills obligate anaerobes on contact. A sample taken to look for them must be shielded from air. It must reach the lab fast, in a special oxygen-free container. Otherwise the germ dies first, and the culture comes back falsely negative.
| Group | Relationship to oxygen | Example |
|---|---|---|
| Obligate aerobe | Requires oxygen | Pseudomonas, M. tuberculosis |
| Obligate anaerobe | Killed by oxygen | Clostridium, Bacteroides |
| Facultative anaerobe | Uses oxygen if present, ferments if not | E. coli, Staphylococcus |
| Microaerophile | Needs low oxygen | Campylobacter, H. pylori |
| Aerotolerant anaerobe | Ignores oxygen but tolerates it | Streptococcus pyogenes, some lactobacilli |
The other axes of classification
Oxygen is only one of several dials. Bacteria are also grouped by the temperature they like. Most human pathogens are mesophiles, happiest near body temperature, 37°C. They are grouped by how much acid (pH) and how much salt they can take. A few extremophiles love strong acid or heavy salt. They are grouped by whether they can live only inside host cells. Two such groups matter a lot. Chlamydia and Rickettsia are obligate intracellular bacteria. Neither can copy itself outside a host cell. So neither grows on a standard plate, and both are found by other tests. Put it all together and you have a coordinate system: shape, Gram reaction, oxygen need, growth needs, and lifestyle. Those coordinates let a lab pin down almost any germ.
- Oxygen sorts bacteria into obligate aerobes, obligate anaerobes, facultative anaerobes, microaerophiles, and aerotolerant anaerobes.
- Anaerobes cause deep wound, abscess, and dead-tissue infections. Their samples must be protected from air in transit.
- Other axes are temperature, pH, salt, and an intracellular life, as in Chlamydia and Rickettsia.
- Combining all the features — shape, stain, oxygen, growth needs — is how the lab names an organism.
The chapter in one breath
- Bacteria are sorted first by shape (coccus, bacillus, spirillum) and arrangement (pairs, chains, clusters). Those clues are usually built into the genus name.
- The Gram stain splits bacteria in two. Gram-positive cells have thick peptidoglycan and stain purple. Gram-negative cells have a thin wall plus an outer membrane and stain pink.
- Key gram-positive pathogens are Staphylococcus (clusters), Streptococcus (chains), and the spore-forming anaerobes Clostridium and Clostridioides.
- Key gram-negative pathogens are E. coli, Salmonella, and Pseudomonas. Their endotoxin, the lipid A of LPS, can drive septic shock.
- Special bacteria break the rules. Mycobacteria are acid-fast, spirochetes are coils too thin to stain, and mycoplasmas have no cell wall, so penicillins fail.
- Oxygen needs — aerobe, anaerobe, facultative, microaerophile — shape where infections form and how samples must be collected.
- More axes finish the coordinate system: temperature, pH, salt, and life inside host cells.
ReferenceWords worth knowing
A quick reference for the terms in this chapter. Skim it now, or come back whenever a word trips you up.
- Acid-fast
- A staining property of bacteria whose waxy, mycolic-acid wall keeps its color when acid-alcohol is used to wash it out. This is how mycobacteria are found.
- Aerobe (obligate)
- A bacterium that must have oxygen to grow, such as Pseudomonas or M. tuberculosis.
- Anaerobe (obligate)
- A bacterium that is killed by oxygen and grows only where oxygen is missing, such as Clostridium.
- Arrangement
- The pattern bacterial cells form after dividing: pairs (diplo-), chains (strepto-), or clusters (staphylo-).
- Bacillus
- A rod-shaped bacterium. The plural is bacilli.
- Catalase
- An enzyme that breaks down hydrogen peroxide. Staphylococci have it and bubble on the bench test; streptococci do not.
- Coagulase
- An enzyme that clots plasma. It is the test that separates S. aureus, which is coagulase-positive, from the other staphylococci.
- Coccus
- A round or ball-shaped bacterium. The plural is cocci.
- Endospore
- A dormant, very tough capsule made by Clostridium and Bacillus. It survives heat, drying, and alcohol.
- Endotoxin
- The lipid A part of the lipopolysaccharide in a gram-negative outer membrane. Released when cells die, it can trigger fever and septic shock.
- Facultative anaerobe
- A bacterium that uses oxygen when it is there but can grow without it, such as E. coli.
- Gram stain
- The staining steps that split bacteria into gram-positive (purple) and gram-negative (pink) by cell wall structure.
- Gram-negative
- Bacteria with a thin peptidoglycan wall and an outer membrane carrying LPS. They stain pink.
- Gram-positive
- Bacteria with a thick peptidoglycan wall that traps crystal violet. They stain purple.
- Hemolysis
- The breakdown of red cells in blood agar around a colony. Beta is a clear zone, alpha is a green zone, and gamma is none. It is how streptococci are grouped.
- Lipopolysaccharide (LPS)
- The molecule on the gram-negative outer membrane. Its lipid A part is endotoxin.
- Microaerophile
- A bacterium that needs oxygen but only at low levels, such as Campylobacter or H. pylori.
- MRSA
- Methicillin-resistant Staphylococcus aureus. A resistant strain that calls for contact precautions.
- Mycolic acid
- The waxy fat in the mycobacterial cell wall. It is the reason these cells are acid-fast and so hardy.
- Peptidoglycan
- The mesh-like layer that forms the bacterial cell wall. It is the target of the penicillin family.
- Pleomorphic
- Having no set shape. It describes wall-less bacteria such as Mycoplasma.
- Spirillum
- A spiral or corkscrew-shaped bacterium. The plural is spirilla.
- Spirochete
- A thin, tightly coiled bacterium that moves using internal axial filaments, such as Treponema pallidum.
- Vibrio
- A short, curved rod shaped like a comma, as in Vibrio cholerae.
Check yourself
Start with the key below. Work a few unknowns through it, then try the written questions.