Introduction

Bacterial pneumonia is a common lung infection that can become serious. It develops after you have been infected with specific types of bacteria.^1,2 Bacterial pneumonia is one of the common types of pneumonia, although other microbes such as viruses and parasites, can also cause it. This disease is considered contagious as the bacteria can spread when you come into contact with bacteria-infected droplets via inhalation.² 

The setting where a person contracted pneumonia can provide important information on the type of organism causing the disease and the necessary management. Pneumonia can be contracted outside of a healthcare setting (Community-acquired pneumonia) or in a healthcare setting as either Hospital-acquired pneumonia (also known as Nosocomial pneumonia) or Ventilator-associated pneumonia.^1,2

To understand how bacteria can impact the lungs and cause this disease, you need to understand the respiratory system first. The respiratory system is made up of the organs that help you to breathe, consisting of two parts: the upper respiratory tract and the lower respiratory tract. The lower respiratory tract is illustrated in Figure 1 – this is typically where bacterial pneumonia typically affects you.

Air enters the body via the upper respiratory tract and then the lower respiratory tract is where oxygen is absorbed and carbon dioxide is released. The lower respiratory tract is also exposed to microbes that may be present in the air, but the body has a natural defence in place to detect and eliminate these microbes (the immune system).

In bacterial pneumonia, the infection in the lungs overwhelms the immune response which triggers a stronger, inflammatory response from the body to prevent the bacteria spreading.^1,2 This response can harm the lungs leading to fluid or pus filling the alveoli.^1,2 This prevents the lungs from functioning properly and results in the symptoms seen in bacterial pneumonia.^1,2 In the longer term, infection can lead to scarring of lung tissue.

There are two types of bacterial pneumonia: typical and atypical. The differences are outlined in Table 1.^1,2

Table 1: A comparison of the characteristics of typical and atypical bacterial pneumonia.^1-4

The inflammation in the lungs can also appear differently in typical and atypical bacterial pneumonia. Accurate diagnostic tools are needed to ensure an individual receives the right treatment for pneumonia. Bacterial pneumonia can be diagnosed via:^1,2

• Understanding the patient’s history
• Physical examination of the patient
• Blood tests to look for signs of inflammation
• Scans such as chest x-rays 

Among the various diagnostic methods, chest X-rays are primarily used to look for signs of infection when someone has symptoms of pneumonia. 

The role of chest X-rays

Chest X-rays, also known as chest radiographs, are a type of imaging where a small amount of radiation is passed through your torso to produce an image of the inside of the chest. This test is useful for diagnosing a range of health conditions. A chest X-ray is not painful. It is quick and non-invasive test where you are only exposed to the X-rays for a brief second. 

In X-ray images, your body appears as shades of black, white, and grey.⁵ This is because X-rays show different parts of the body based on their density.⁵ The denser the body part, the more white it appears on the image, and vice versa. Therefore, in chest X-rays, the bones that make up the ribcage are white whereas the lungs, which are filled with air, appear black.⁵ In pneumonia, however, as the inflammation in the lungs cause denser substances (such as fluid and pus) to fill the lungs, so areas of the lungs appear grey or white on chest X-ray images (known as consolidation).⁵ 

Common X-ray findings in bacterial pneumonia

Bacterial pneumonia can affect different parts of the lungs and, as some bacteria are known to target specific areas, this can help in diagnosing the disease.² Some of the differences in X-ray appearances are described below:

• A chest X-ray image where one lung lobe is almost completely grey/white

This type of image is frequently seen in typical bacterial pneumonia, such as Streptococcus pneumoniae, but less commonly with atypical pneumonia.¹

• A chest X-ray image where both lung lobes are almost completely grey/white
• A chest X-ray image where patchy grey/white areas appear across either one or both lobes and the bronchi appear inflamed. This type of image can be seen with atypical bacteria such as Chlamydophila pneumoniae³

Chest X-rays are also used for disease monitoring. A repeat chest X-ray can be performed to determine whether the given treatment has been successful in clearing the infection.

Limitations of chest x-rays in diagnosing bacterial pneumonia

Although chest X-rays are recommended to diagnose someone with bacterial pneumonia, there are limitations.

There are occasions where an individual may have symptoms of the disease but they have negative chest X-ray findings.⁶ For example, negative chest X-rays have been reported in cases of atypical bacterial pneumonia caused by Mycoplasma pneumoniae.

Chest x-ray interpretation can be limited by the projection of the image, especially in elderly patients, which may lead to misdiagnosis.⁷ Chest x-rays are also not specific at diagnosing pneumonia as consolidation can be caused by other lung conditions.^3,8 

Alternatives to chest X-rays

Chest x-rays are the primary investigation for diagnosing pneumonia. Ultrasound of the lungs can be performed in the emergency department to rule out life-threatening conditions, but signs of infection can occasionally be seen. In certain cases, for example if a clot on the lungs (pulmonary embolism) is suspected, a CT scan may be performed. CT is more sensitive for detecting infection on the lungs.⁹ However, it comes with the downside of exposure to ionising radiation, and in most cases, chest X-ray alone is sufficient. A specialist hospital doctor may request a CT scan if the infection has not cleared with antibiotics or another diagnosis is suspected. 

FAQs

What should I expect when I go for a chest X-ray?

Chest X-rays are performed at a healthcare facility. When you go, you will be asked to remove your clothing and anything else (e.g. jewellery) that could interfere with the test. You will be given a gown and will be asked to stand, or sometimes sit or lie down, for the x-ray machine to be positioned on your chest so the images can be taken.

How is bacterial pneumonia treated after diagnosis?

After diagnosis, the treatment options can vary. Some of the options are explained below.

1. Specific antibiotics are given to patients based on the bacteria known to be causing the disease²
2. Medication given to manage the symptoms of the condition such as paracetamol to reduce a fever²

There are also vaccines available for some of the bacteria that cause the disease and these can reduce the risk of an individual developing bacterial pneumonia.

Summary

Bacterial pneumonia is an infection where the body’s immune response to the presence of bacteria leads to inflammation and the accumulation of fluid in the lungs. The symptoms of the condition can become serious and it needs to be diagnosed swiftly for adequate treatment. Chest x-rays are very useful in diagnosing bacterial pneumonia and can provide information on the location of the infection in the lungs, with a few limitations. They are a quick, easy, and painless imaging technique. Chest X-rays are the mainstay for diagnosing bacterial pneumonia both in hospital and in community settings.