肺炎克雷伯菌隶属于肠杆菌科、克雷伯菌属，是一种革兰氏阴性、兼性厌氧杆菌。有厚厚的荚膜，无芽孢和鞭毛，通常定植于人体粘膜表面，特别是口鼻黏膜以及胃肠道黏膜。肺炎克雷伯菌可以在土壤、水、多种植物、昆虫、鸟类、爬行动物以及哺乳动物等多种生态位中以共生菌或潜在病原体的形式存在。
全球大多数的肺炎克雷伯菌的感染是机会性医院性感染（HAI）。最常见的临床表现为肺炎、尿路和伤口感染，其中任何一种感染都可能发展为菌血症。在医院环境外，肺炎克雷伯菌可表现为一种“真正的”病原体，引发严重的社区获得性感染（CAIs）。常见的CAIs包括眼内炎、肺炎、坏死性筋膜炎、非肝源性脓肿、脑膜炎以及无胆道疾病情况下的化脓性肝脓肿。在不寻常和/或多个部位引起感染被认为是高毒力肺炎克雷伯菌感染的临床表现，其通常伴随菌血症和/或转移性扩散。世界卫生组织将产广谱β-内酰胺酶（ESBL）和耐碳青霉烯类的肺炎克雷伯菌（CRKp）视为严重的公共卫生威胁。据报道，仅欧洲此类菌株每年能造成超过90,000例感染病例和7,000多人死亡。
肺炎克雷伯菌中耐药性最强的谱系（例如，对第三代头孢菌素和/或碳青霉烯耐药）已成为全球性公共卫生难题，这些克隆群包括CG258、CG15、CG20（CG17）、CG29、CG37、CG147、CG101（CG43）和 CG307，它们彼此间没有亲缘关系，但均呈现广泛的地理分布，并作为常见诱因导致多重耐药HAI和/或暴发事件。肺炎克雷伯菌对氨苄西林具有固有耐药性，其耐药机制是通过染色体核心基因blaSHV编码产生的A类β-内酰胺酶SHV实现。该酶可水解氨苄西林等青霉素类抗生素，使菌株天然耐受此类药物。外排泵OqxAB和AcrAB的表达和/或活性的变化与对多种抗生素的耐药性有关，如氟喹诺酮类、呋喃妥因、替加环素、氯霉素和碳青霉烯类药物。
高毒力肺炎克雷伯菌感染常见于CG23、CG65（包括ST65和ST375）和CG86。高毒力克隆通常携带核心致病因子毒力相关变异体（K1/K2荚膜、O1/O2 LPS）以及辅助毒力因子（rmpA和/或 rmpA2，获得性铁载体ybt、iuc和iro)。极少数情况下，非K1或非K2荚膜（例如K5、K20、K54和K57）和/或不含辅助毒力因子的菌株也会导致高毒力感染。
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