Streptococcus pneumoniae

Streptococcus pneumoniae is a group of Gram-positive bacteria that appear oval-shaped, lanceolate, or kidney-like; they are relatively large, measuring 0.5 to 1.25 μm in diameter, and typically form characteristic pairs (diplococci). First isolated in 1881, it was initially named Pneumococcus. In 1920, the bacterium was renamed Diplococcus pneumoniae. However, due to its striking resemblance to streptococci, it was officially reclassified as Streptococcus pneumoniae in 1974. In 1923, Frederick Griffith (1879–1941), working at the Pathological Laboratory of the British Ministry of Health, discovered that pneumococcal colonies exist in two distinct types:

One type is smooth and dome-shaped, with a relatively regular form. This bacterium has a capsule and is referred to as the smooth type (Smooth, S-type), which can cause pneumonia in humans, septicemia in mice, and even death.

Another type of colony is granular, irregular, and lacks a capsule, known as the rough (Rough, r) type (which does not cause death in mice). Meanwhile, the S-type, encapsulated Streptococcus pneumoniae, is the primary strain tested in experimental animals.

It is primarily transmitted via aerosols or by contact with nasal or tear gland secretions from infected animals.

Among pyogenic cocci, Streptococcus pneumoniae ranks second only to Staphylococcus aureus in terms of pathogenicity. Notably, however, Streptococcus pneumoniae has so far shown extremely rare resistance to penicillin-class antibiotics. The primary virulence factors of pneumococcus are pneumolysin and the capsule—both of which are key contributors to its pathogenicity. Interestingly, the capsule is antigenic and serves as the basis for classifying different serotypes of Streptococcus pneumoniae.

Streptococcus pneumoniae is a zoonotic bacterium that can colonize the upper respiratory tracts of both humans and animals. Healthy individuals often carry this bacterium at high rates, typically experiencing asymptomatic infections. It’s a common opportunistic pathogen, found in the nasopharynx of approximately 40% of healthy people, and can lead to serious conditions such as pneumonia, otitis media, bacteremia, and meningitis—diseases associated with significant morbidity and mortality worldwide. Guinea pigs and rats are particularly susceptible. Most strains of this bacterium are either non-pathogenic or cause only mild illness, frequently resulting in silent, latent infections.

Rats: Disheveled fur, arched backs, huddling in cage corners, difficulty breathing, loss of appetite, or sudden death—sometimes accompanied by specific symptoms such as runny nose, blood-streaked nostrils, conjunctivitis, and vestibular disorders.

Guinea pigs exhibit disheveled fur, lethargy, and reduced appetite, along with pale mucous membranes, coughing, and labored breathing accompanied by crackling sounds. Fluid or purulent discharge may also be observed from the eyes and nose. In pregnant females, this can lead to miscarriage, birth of deformed offspring, or stillbirths. After giving birth, mothers often appear weak and listless—and in severe cases, may even die.

When pneumonia is caused by this organism: patients typically experience high fever, chills, severe pleuritic chest pain, and cough up rust-colored sputum. If the infection spreads to other parts of the body, it can lead to secondary conditions such as secondary pleurisy, otitis media, mastoiditis, endocarditis, and purulent meningitis.

Lungs: Congestion, enlargement.

Nasal cavity and trachea: Both are filled with mucus and scattered blood streaks.

Nasal cavity and tympanic cavity: Serous to purulent exudates were observed.

Lungs: Prominent areas of solid lesions with deepened red coloration can be observed.

Chest cavity: Minimal fluid accumulation, with evident pleuritis and pericarditis (observed in deceased rats).

Under microscopic examination, the alveolar septal vessels show dilation and congestion, with infiltration of lymphocytes and plasma cells, consistent primarily with chronic interstitial pneumonia. Additionally, pulmonary emphysema is present, while the trachea exhibits predominantly chronic specific inflammation.

To prevent the spread of Streptococcus pneumoniae, the animal housing room must maintain stable temperature and humidity levels—avoiding extreme fluctuations—and ensure that animal density remains appropriately low. Additionally, the room should be well-ventilated with consistently clean air quality. When acquiring animals from outside sources, it’s crucial to ensure they arrive under optimal transportation conditions and with suitable vehicles. Given current advancements, animals should ideally be housed within either an enhanced biocontainment system or a standard barrier system, which effectively minimizes disease outbreaks. Furthermore, any sick animals should be promptly isolated or culled, while newly introduced animals must undergo quarantine and thorough health checks. Since Streptococcus pneumoniae can potentially originate from humans, animal care staff are advised to wear masks and other standard personal protective equipment to reduce the risk of infection. Individuals diagnosed with or suspected of having Streptococcus pneumoniae-related illnesses—such as pneumonia, otitis media, conjunctivitis, or other confirmed or possible infections—should refrain from handling animals until their antibiotic treatment has fully concluded. Implementing these preventive measures not only safeguards the health of the animals but also protects humans from potential transmission of Streptococcus pneumoniae.

Pneumococcus is sensitive to common disinfectants used in animal facilities. Any chemical or physical disinfection method can effectively eliminate pneumococcus from the environment. While antibiotics can treat the infection—typically making the bacteria susceptible to antibiotics like penicillin and sulfonamides—they do not address carriers of the pathogen, nor can they eradicate bacteria residing on bedding or cage surfaces. Therefore, antibiotic treatment is recommended only to alleviate clinical symptoms. Human-isolated strains of pneumococcus often exhibit multidrug resistance. To establish a pneumococcus-free population, animals should be obtained through embryo transfer or hysterectomy.

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