Pneumonia is a disease which affects the parenchyma of the lungs and can be caused by bacteria, viruses or fungi, with bacteria being the most common cause. It can be very serious and potentially fatal in patients with pre-existing heart and lung conditions. Bacterial pneumonia is usually treated with antibiotics, depending on the type of bacteria which caused the infection. Antibiotics should not be used to treat viral or most fungal pneumonia infections.¹

What is bacterial pneumonia?

Bacterial pneumonia causes inflammation in the lung parenchyma and alveoli. It can usually be treated quickly, but some rare cases can be serious and have long-term effects on lung function and quality of life.

Symptoms of bacterial pneumonia

• Fever
• Coughing
• Chest pain
• Shortness of breath

Severe complications associated with bacterial pneumonia include: necrotising pneumonia, meningitis, sepsis and multiple organ failure.²

Types of bacteria

Bacteria are divided into two types: gram-positive and gram-negative. Gram-positive bacteria have a thick cell wall made up of a protein called peptidoglycan, while gram-negative bacteria have an outer membrane made up of lipopolysaccharides, as well as a much thinner peptidoglycan cell wall.³

The most common bacteria that cause pneumonia are: Streptococcus pneumoniae (gram-positive), Haemophilus influenzae (gram-negative) and Staphylococcus aureus (gram-positive). These bacteria can be treated using different types of antibiotics.²

How antibiotics work

Antibiotics can either be bacteriostatic or bactericidal. Bacteriostatic antibiotics prevent bacteria from growing further, while bactericidal antibiotics kill bacteria.⁴ Different types of antibiotics work by targeting different processes in the bacterial cell cycle. Some inhibit cell wall synthesis, others inhibit nucleic acid (e.g. DNA) synthesis and other target protein synthesis.⁵ 

Antibiotics used to treat bacterial pneumonia

The antibiotic prescribed to treat each case of bacterial pneumonia usually varies depending on the type of bacteria and how susceptible or resistant they are to each antibiotic. Certain antibiotics will also not be prescribed to patients if they are allergic to penicillin.

Amoxicillin

Amoxicillin is a bactericidal antibiotic derived from penicillin. It is effective against bacteria belonging to the Streptococcus and Staphylococcus species and Haemophilus influenzae. It belongs to a class of antibiotics known as ‘Beta-lactam antibiotics’. The beta-lactam ring is a highly reactive molecular structure which is a component of 65% of antibiotics currently on the market.⁶ Amoxicillin kills bacteria by activating enzymes that break down the cell wall of the bacterium. It is the most commonly prescribed antibiotic in cases of bacterial pneumonia. Common side effects of amoxicillin include nausea, vomiting and diarrhoea. Some rare, but serious adverse effects may be: kidney stones, interstitial nephritis and allergic reactions.⁷

Macrolides

Macrolides are a class of bacteriostatic antibiotics which are used to treat bacterial pneumonia, sometimes as an alternative to amoxicillin if the patient is allergic to penicillin. However, they usually aren’t prescribed to treat pneumonia caused by Streptococcus pneumoniae as this bacteria is highly resistant to macrolides. Some examples include: azithromycin, clarithromycin and erythromycin. They work by inhibiting protein synthesis in bacteria so that they are unable to multiply, making it easier for immune cells to kill them and control the infection. 

Use of macrolides is associated with common side effects such as nausea, vomiting, abdominal pain and diarrhoea. They can also sometimes increase the patient’s risk of developing heart arrhythmia. Some patients have also experienced slight hearing loss, which has been reversed after they stopped taking the drug.⁸

Fluoroquinolones

Fluoroquinolones are a bactericidal class of antibiotics often used to treat respiratory tract infections such as pneumonia. They are effective against both gram-positive and gram-negative bacteria. Examples include: ciprofloxacin, levofloxacin, moxifloxacin, ofloxacin and delafloxacin. They are associated with many adverse effects, and some have been withdrawn from the market due to severe, occasionally fatal complications. Some examples of adverse effects include: rashes, allergic reactions, tendinitis, heart arrhythmias, hypoglycemia and hyperglycemia.⁹ 

Fluoroquinolones target two enzymes involved in bacterial DNA synthesis: DNA gyrase and topoisomerase IV. Fluoroquinolones inhibit the actions of these enzymes which damage DNA, leading to the death of the bacteria.¹⁰

Tetracyclines

Tetracyclines are a bacteriostatic class of antibiotics which is commonly used to treat respiratory tract infections, including pneumonia, in adults. They are normally prescribed after beta-lactams, macrolides and fluoroquinolones have been considered and have a broad spectrum of action; they are effective against many gram-positive and gram-negative bacteria, as well as some atypical pneumonia-causing pathogens. Doxycycline is the most commonly prescribed tetracycline, either on its own or in combination with a beta-lactam antibiotic. 

Tetracyclines work similarly to macrolides in that they inhibit protein synthesis and stop bacteria from multiplying. Common side effects associated with tetracyclines in general are: nausea, diarrhoea, vomiting, oesophagitis, photosensitivity, lightheadedness and dizziness.¹¹

Antibiotic resistance

Bacterial resistance to antibiotics is a current global health threat which is associated with high levels of mortality, particularly in hospital environments, though resistant bacteria are now being found in community settings more often than they were before. The World Health Organisation (WHO) deems antibiotic resistance to be one of the top three most prominent public health threats in the 21st century. This is due to antibiotics being prescribed when they aren’t required and improper usage by patients. In recent years, the number of bacteria that have become resistant to multiple antibiotics has been increasing at alarming rates.¹²

Antibiotic resistance is the result of natural selection, and bacteria evolving to overcome the actions of antibiotic drugs. Bacteria reproduce very quickly; approximately once every 20 minutes. This therefore also allows bacteria to evolve much quicker than most organisms. Mutations in genes that can allow antibiotic resistance occur randomly. Then, bacteria can transfer genes to one another through a process called horizontal gene transfer (HGT), allowing more bacteria with the resistant gene to reproduce. Resistance to almost every antibiotic that has been developed has been observed, usually within 10 years following the introduction of the antibiotic.

It is crucial that when prescribed antibiotics, you take the full course, even if symptoms clear up early. Antibiotics will quickly kill bacteria which are sensitive to the drug but will leave behind resistant bacteria. So even if you don’t have any symptoms, there may still be antibiotic-resistant bacteria present in your body which, if you stop taking your antibiotics early, will be able to survive and reproduce, creating more resistant bacteria which antibiotics are ineffective against.¹³

Summary

Pneumonia is a disease which causes inflammation of the lung parenchyma and alveoli. It is most commonly caused by bacteria, but can also be caused by viruses and fungi. It normally clears up quickly with treatment, but can be serious and potentially life-threatening in some patients. Bacterial pneumonia can be caused by many different bacteria, such as Streptococcus pneumoniae, Haemophilus influenzae and Staphylococcus aureus.

The most common treatment for bacterial pneumonia is antibiotics such as amoxicillin, macrolides, fluoroquinolones and tetracyclines. Most of these antibiotics have typical side effects such as nausea, diarrhoea and vomiting, but some can have potentially more serious adverse effects. Antibiotics are prescribed depending on the type of bacteria that is causing the illness and if the bacteria is resistant to certain antibiotics. Antibiotic-resistant bacteria are a severe threat to global health and can be avoided by patients ensuring they take the entire course of antibiotics that they are prescribed, even after the symptoms of their illness are gone.