Skip to Content

What is gram-positive look like?

Gram-positive bacteria are a group of bacteria that have a thick peptidoglycan cell wall that retains crystal violet dye when stained. This gives them a purple color under the microscope when a Gram stain procedure is performed. Some of the most common gram-positive bacteria include Staphylococcus, Streptococcus, Bacillus, Listeria, Clostridium, and Actinomyces. Gram-positive bacteria can cause a variety of infections and are a major cause of hospital-acquired infections. Understanding the characteristics and visual appearance of gram-positive bacteria is important for identification and diagnosis.

Gram Staining Procedure

The Gram stain procedure distinguishes between gram-positive and gram-negative bacteria based on differences in their cell wall structure. The steps are:

  1. Apply primary stain (crystal violet) to a heat-fixed smear of bacteria – both gram-positive and gram-negative bacteria will stain purple.
  2. Add iodine mordant – iodine forms a complex with crystal violet to help retain the primary stain.
  3. Decolorize with alcohol – the thin peptidoglycan layer of gram-negative bacteria is easily decolorized while the thick layer in gram-positive bacteria retains crystal violet.
  4. Counterstain with safranin – gram-negative bacteria will take up the pink counterstain while gram-positive bacteria remain purple.

This staining procedure results in gram-positive bacteria appearing purple under the microscope while gram-negative bacteria appear pink.

Appearance of Gram-Positive Bacteria

Here are some key features in the microscopic appearance of gram-positive bacteria:

  • Retain crystal violet stain and appear purple or blue-purple.
  • Cell morphology can be spherical (cocci) or rod-shaped (bacilli). Some form chains or clusters.
  • Thick peptidoglycan cell wall gives the cells a rigid, smooth appearance with defined edges.
  • Lack outer membrane found in gram-negative bacteria.
  • Common arrangements include streptococci (chains), staphylococci (grape-like clusters), and bacilli (single rods).
  • Some forms spores which appear as oval, refractile bodies inside the cell.

Some examples:

Bacteria Cell Morphology Arrangement
Staphylococcus aureus Cocci Grape-like clusters
Streptococcus pyogenes Cocci Chains
Bacillus anthracis Bacilli Single rods
Clostridium perfringens Bacilli Chains

Comparison to Gram-Negative Bacteria

Gram-positive and gram-negative bacteria can be distinguished by the following microscopic characteristics:

Feature Gram-Positive Bacteria Gram-Negative Bacteria
Gram stain result Purple Pink
Cell wall thickness Thick peptidoglycan layer Thin peptidoglycan layer
Outer membrane Absent Present
Cell morphology Cocci or bacilli Usually bacilli or curved rods
Arrangement Clusters, chains, single cells Usually single rods

Gram-negative bacteria have an outer membrane that contains lipopolysaccharide and functions as an endotoxin. This outer membrane makes their cell wall more complex compared to gram-positive bacteria.

Major Groups of Gram-Positive Bacteria

There are several medically important genera of gram-positive bacteria. Here is an overview of their basic morphology and arrangements:


  • Cocci that appear in grape-like clusters.
  • Important species include S. aureus and S. epidermidis.
  • S. aureus causes skin infections, pneumonia, and food poisoning.
  • S. epidermidis is a common cause of hospital-acquired infections associated with indwelling medical devices.


  • Spherical cocci that form chains.
  • Important pathogenic species include S. pyogenes, S. pneumoniae, S. agalactiae.
  • S. pyogenes causes strep throat, skin infections.
  • S. pneumoniae causes pneumonia, meningitis, sinusitis.
  • S. agalactiae causes neonatal meningitis.


  • Cocci that form pairs or short chains.
  • Includes E. faecalis and E. faecium.
  • Normal gut flora but can cause UTIs, endocarditis, bacteremia.
  • Resistant to multiple antibiotics.


  • Rod-shaped bacilli that may form chains or filaments.
  • Aerobic, spore-forming bacteria found widely in soil.
  • B. anthracis causes anthrax.
  • B. cereus causes food poisoning.


  • Pleomorphic rods that may form long chains.
  • Obligate anaerobes, form highly resistant spores.
  • C. tetani causes tetanus.
  • C. perfringens causes gas gangrene and food poisoning.
  • C. difficile causes antibiotic-associated diarrhea.


  • Small rods with a tumbling motility pattern.
  • L. monocytogenes causes listeriosis, a foodborne illness.
  • Infects immunocompromised individuals, pregnant women.


  • Pleomorphic, club-shaped rods.
  • Part of normal skin flora.
  • C. diphtheriae causes the disease diphtheria.


  • Branching, filamentous rods that resemble fungal hyphae.
  • Normal oral flora, can cause actinomycosis.
  • Slow-growing, facultative anaerobes.

Clinical Importance

Gram-positive bacteria are responsible for a wide variety of human diseases. Some key examples include:

  • Staphylococcus aureus – skin and soft tissue infections, pneumonia, food poisoning, toxic shock syndrome
  • Streptococcus pyogenes – strep throat, impetigo, necrotizing fasciitis
  • Streptococcus pneumoniae – pneumonia, meningitis, otitis media
  • Enterococcus faecalis – urinary tract infections, endocarditis, bacteremia
  • Clostridium difficile – antibiotic-associated diarrhea
  • Clostridium perfringens – gas gangrene, food poisoning
  • Bacillus anthracis – anthrax
  • Listeria monocytogenes – listeriosis
  • Corynebacterium diphtheriae – diphtheria

Gram-positive bacteria are a frequent cause of hospital-acquired infections. The ability of many gram-positive species to form biofilms on implanted medical devices makes them a major concern. In addition, multidrug resistant gram-positive bacteria such as MRSA (methicillin-resistant S. aureus), VRE (vancomycin-resistant enterococci), and VRSA (vancomycin-resistant S. aureus) are extremely difficult to treat and can cause outbreaks in healthcare facilities.


Gram-positive bacteria are characterized by their purple coloration under the Gram stain procedure. They have a thick peptidoglycan cell wall that retains the crystal violet dye. Their morphology can be cocci or bacilli, and they often form characteristic arrangements like clusters, chains, or filaments. Medically important gram-positive bacteria include Staphylococcus, Streptococcus, Enterococcus, Clostridium, Bacillus, Listeria, and Corynebacterium among others. These bacteria cause a wide array of diseases and are common sources of drug-resistant hospital-acquired infections. Understanding the visual appearance and structural features of gram-positive bacteria is key for their laboratory identification and diagnosis of associated infections.