In the bloodstream, the bacteria can infect both phagocytes and non-phagocytes. B. pseudomallei use their flagella to move near host cells, then attach to the cells using various adhesion proteins, including the type IV pilus protein PilA as well as adhesion proteins BoaA and BoaB. Additionally, adhesion of the bacteria partially depends on the presence of the host protein Protease-activated receptor-1 which is present on the surface of endothelial cells, platelets, and monocytes. Once bound, the bacteria enter host cells through endocytosis, ending up inside an endocytic vesicle. As the vesicle acidifies, B. pseudomallei uses its Type 3 secretion system (T3SS) to inject effector proteins into the host cell, disrupting the vesicle and allowing the bacteria to escape into the host cytoplasm. Within the host cytoplasm, the bacteria evade being killed by the host autophagy using various T3SS effector proteins. The bacteria replicate in the host cytoplasm.Inside the host cell, the bacteria move by inducing the polymerization of the host actin behind them, propelling the bacteria forward. This actin-mediated motility is accomplished with the autotransporter BimA which interacts with actin at the tail-end of the bacterium. The bacteria that has BimABm allele has higher possibility of causing neurological melioidosis, thus higher chance of death and residual disability to the host when compared to the bacteria that has BimABp variant. Propelled by actin, the bacteria push against the host membrane, creating protrusions that extend into neighbouring cells. These protrusions cause neighboring cells to fuse, leading to the formation of multinucleated giant cells (MNGCs).
The clinical manifestation of the disease can range from simple skin changes such as abscesses or ulcerations to severe organ problems. The commonest organs affected are liver, spleen, lungs, prostate, and kidneys. Among the most common features are bacteremia (in 40 to 60% of cases), pneumonia (50%), and septic shock (20%). People with only pneumonia may have a prominent cough with sputum and shortness of breath. However, those with septic shock together with pneumonia may have minimal coughing. Results of a chest X-ray can range from diffuse nodular infiltrates in those with septic shock to progressive consolidation located most commonly in the upper lobes for those with pneumonia only. Pleural effusion and empyema are more common for melioidosis affecting lower lobes of the lungs. In 10% of cases, people develop secondary pneumonia caused by other bacteria after the primary infection. In northern Australia, 60% of the infected children presented with only skin lesions, while 20% presented with pneumonia.Depending on the course of infection, other severe manifestations develop. Approximately 1 to 5% of those infected develop inflammation of the brain and brain covering or brain abscess; 14 to 28% develop pyelonephritis, kidney abscess or prostatic abscesses; 0 to 30% develop neck or salivary gland abscesses; 10 to 33% develop liver, spleen, or paraintestinal abscesses; and 4 to 14% develop septic arthritis and osteomyelitis. Rare manifestations include lymph node disease resembling tuberculosis, mediastinal masses, pericardial effusion, mycotic aneurysm, and inflammation of the pancreas.