Understanding What is the Function of Penstrep?

October 6, 2025 •

4 min read

Penicillin-Streptomycin, or Penstrep, was first introduced for use in cell culture in 1955 and has since become one of the most common antibiotic solutions for preventing bacterial contamination. The key to understanding what is the function of penstrep lies in its dual-component composition, which targets a wide range of bacteria.

Quick Summary

Penstrep is a combination antibiotic solution composed of penicillin and streptomycin. Its function is to provide broad-spectrum antimicrobial activity by targeting both Gram-positive and Gram-negative bacteria, primarily for use in cell culture and veterinary treatments.

Key Points

Dual Action: Penstrep combines penicillin and streptomycin to provide a broad-spectrum antibiotic effect, targeting both Gram-positive and Gram-negative bacteria.
Cell Wall and Protein Synthesis: Penicillin inhibits cell wall synthesis, while streptomycin inhibits protein synthesis by binding to bacterial ribosomes.
Cell Culture Application: A primary function is to prevent bacterial contamination in mammalian cell culture without harming the cells.
Veterinary Use: It is widely used in veterinary medicine for treating systemic bacterial infections in livestock like cattle, horses, pigs, and sheep.
Synergistic Effect: The combination is more effective than either antibiotic alone because penicillin's action can enhance the uptake of streptomycin.
Resistance Concerns: The overuse of Penstrep, like other antibiotics, contributes to the development of antibiotic-resistant bacterial strains.
Potential Side Effects: In live animals, Penstrep can cause allergic reactions (penicillin) and toxicities like hearing loss or kidney damage with prolonged use (streptomycin).

What is Penstrep? The Synergistic Duo

Penstrep is not a single drug but a powerful antimicrobial cocktail combining two distinct antibiotics: penicillin G and streptomycin sulfate. This mixture is designed to leverage the individual strengths of its components to achieve a broader spectrum of antibacterial coverage than either could alone. Penicillin is a beta-lactam antibiotic, historically significant for its discovery in 1928, and is effective primarily against Gram-positive bacteria. Streptomycin, an aminoglycoside discovered in the 1940s, is effective against many Gram-negative bacteria and is notable for its role in treating diseases like tuberculosis. By combining these two, Penstrep offers a robust defense against a wider variety of potential bacterial invaders.

The Dual Mechanism of Action

The efficacy of Penstrep comes from its two-pronged attack on bacterial cells. The two components disrupt different, but critical, cellular processes, leading to a synergistic effect that is often more potent than the sum of its parts.

1. Penicillin: Disrupting the Cell Wall

Target: Penicillin’s primary target is the bacterial cell wall.
Process: It inhibits the synthesis of peptidoglycan, a key structural component of the cell wall, particularly in Gram-positive bacteria.
Outcome: Without a stable cell wall, the bacterial cell's internal pressure causes it to burst (lyse), killing the cell.

2. Streptomycin: Inhibiting Protein Synthesis

Target: Streptomycin targets the bacterial ribosome, specifically the 16S portion of the 30S ribosomal subunit.
Process: By binding to this site, it causes the ribosome to misread messenger RNA, leading to the synthesis of faulty, non-functional proteins.
Outcome: The accumulation of these incorrect proteins is toxic to the cell, ultimately inhibiting its growth and leading to cell death.

3. The Synergistic Effect A 1962 study in Science demonstrated that pre-treatment with penicillin enhances the killing effect of streptomycin. This occurs because penicillin damages the bacterial cell membrane, which promotes the uptake of streptomycin into the cell where it can access its ribosomal targets. This synergistic action is particularly valuable for controlling mixed infections involving both Gram-positive and Gram-negative organisms.

Applications of Penstrep

Penstrep’s broad-spectrum antimicrobial activity makes it indispensable in several professional contexts.

In Laboratory Cell Culture

Contamination Control: Penstrep is the most common antibiotic solution used in cell culture media to prevent bacterial contamination.
Mammalian Cell Safety: It is effective at controlling bacterial growth at concentrations that do not cause adverse effects on mammalian cells, making it a safe choice for maintaining sterile cultures.
Common Use: Typical concentrations range from 50 to 100 units/ml for penicillin and 50 to 100 µg/ml for streptomycin, often added as a 1% stock solution.

In Veterinary Medicine

Treating Systemic Infections: In veterinary practice, Penstrep is used to treat systemic infections caused by organisms sensitive to penicillin and/or streptomycin.
Livestock Treatment: It is commonly administered via deep intramuscular injection to cattle, horses, pigs, and sheep to treat conditions like mastitis, enteritis, and respiratory infections.
Viral Co-infections: Penstrep can also be used to control secondary bacterial infections that may arise alongside primary viral diseases.

Comparison with Other Antibiotics

While Penstrep is a reliable and inexpensive option, other antibiotics are sometimes used depending on the specific requirements of the application. The choice often depends on the type of bacteria being targeted, the pH stability needed, and the specific cell lines or animal being treated.


Feature	Penstrep	Gentamicin	Kanamycin
Antibiotic Class	Beta-lactam & Aminoglycoside	Aminoglycoside	Aminoglycoside
Spectrum	Broad-spectrum (Gram+ and Gram-)	Broad-spectrum (Gram+ and Gram-)	Broad-spectrum (Gram+ and Gram-)
Key Mechanism	Cell wall synthesis inhibition (penicillin) and protein synthesis inhibition (streptomycin)	Protein synthesis inhibition	Protein synthesis inhibition
pH Stability	Neutral pH environments are preferred	More stable at lower pH	More stable at lower pH
Cost	Relatively inexpensive	Generally higher	Generally higher
Primary Use	Cell culture, veterinary medicine	Cell culture, veterinary medicine	Cell culture

Risks and Considerations: Navigating the Challenges

As with all antibiotic use, Penstrep is not without its risks and requires careful management. The primary concerns revolve around resistance and potential side effects.

Antibiotic Resistance

Overuse and misuse of antibiotics are major drivers of resistance.
Some bacteria, particularly Streptococcus pneumoniae, have shown dramatically increasing resistance to penicillin over time.
The presence of resistant strains limits treatment options and poses a serious public health threat.

Side Effects

Penicillin: Common side effects in live organisms can include allergic reactions (e.g., skin rashes, anaphylaxis), gastrointestinal issues (nausea, diarrhea), and potential overgrowth of other bacteria like C. difficile.
Streptomycin: As an aminoglycoside, high or prolonged doses of streptomycin can cause ototoxicity (damage to the ear, leading to hearing loss) and nephrotoxicity (kidney damage).
Hypersensitivity: Both components can cause allergic reactions, with hypersensitivity to penicillins and cephalosporins being a known contraindication in animals.

Conclusion

In summary, the function of Penstrep is to act as a broad-spectrum antimicrobial agent by combining the actions of penicillin and streptomycin. This combination leverages penicillin's inhibition of cell wall synthesis in Gram-positive bacteria and streptomycin's inhibition of protein synthesis in Gram-negative bacteria. This dual mechanism makes Penstrep highly effective for preventing bacterial contamination in cell culture and for treating a variety of infections in veterinary medicine. While its use is widespread due to its cost-effectiveness and broad coverage, the risk of antibiotic resistance and potential side effects in living organisms necessitates careful and responsible application.

Learn more about the general considerations for using antibiotics in research and medicine from the CDC: Antibiotic Resistance Threats in the United States

Frequently Asked Questions

Penstrep works by combining two antibiotics with different mechanisms. Penicillin inhibits bacterial cell wall synthesis, while streptomycin inhibits bacterial protein synthesis. This dual action provides a broader range of antibacterial activity against both Gram-positive and Gram-negative bacteria.

In a lab setting, the primary use of penstrep is to supplement cell culture media to prevent bacterial contamination. It is the most common antibiotic solution used for this purpose in mammalian cell culture.

No, penstrep is not intended for human use. Its primary applications are in laboratory cell culture and veterinary medicine. Individual penicillin and streptomycin are used separately for human treatment, but the combination product, especially the form used in labs, is not approved for human therapeutic purposes.

Yes, penstrep is commonly used in veterinary medicine to treat systemic infections in various livestock, including cattle, horses, pigs, and sheep. It is effective against a range of infections caused by bacteria sensitive to either penicillin and/or streptomycin.

In live animals, penstrep can cause side effects. Penicillin can lead to allergic reactions, while streptomycin, particularly with prolonged use, can cause toxicities such as ototoxicity (hearing problems) and nephrotoxicity (kidney damage).

The combination is used to achieve a broader spectrum of activity. Penicillin primarily targets Gram-positive bacteria, while streptomycin is more effective against Gram-negative bacteria. Using them together provides a more comprehensive defense against a wider range of potential bacterial contaminants or pathogens.

Yes, as with other antibiotics, resistance is a major concern. Overuse can lead to the development of resistant bacterial strains, reducing its effectiveness over time. This has been particularly observed with penicillin resistance in some bacteria.