克雷白氏肺炎桿菌是株伺機性感染菌株。它是常見的醫院感染病源，會造成病人尿道感染、肺炎及下腹部感染。在臺灣，克雷白氏肺炎桿菌的感染常造成肝膿瘍，而在西方國家，肝膿瘍卻是由多種微生物所造成，包括大腸桿菌、鍵球菌及一些兼性好氧菌。在臺灣，克雷白氏肺炎桿菌感染所造成肝膿瘍的病患，本身沒有肝病病史，但大部份都有糖尿病病史。在過去的二十年，克雷白氏肺炎桿菌造成的肝膿瘍已成為臺灣特有的疾病。
在過去的研究中，我們選殖到一個克雷白氏肺炎桿菌基因omp2，其基因產物的胺基酸序列與Coxiella burnetii的外膜蛋白質COM1有很高的相似性. 而Coxiella burnetii 的這個外膜蛋白被認為是一個很重要的免疫系統辨識因子, 並且已發展為疫苗來保護牛隻免於受到Coxiella burnetii的感染。研究Omp2是否會在克雷白氏肺炎桿菌感染過程中，引發免疫反應，我們將Omp2當成疫苗來用，在動物實驗中，測試Omp2能否產生保護小鼠免於受到克雷白氏肺炎桿菌的感染。但結果卻未如我們所預期的有任何保護的效果。
然而在胺基酸的序列比對上，我們發現Omp2上帶有一個區域與DsbA的活化中心相似。在大腸桿菌中，DsbA在膜間隙是主要的氧化催化劑，它影響著雙硫鍵的形成。而dsbA 的突變會造成菌外表型上，很多的改變，這些外表型包括鹼性磷酸酵素及脂酵素的活性下降，會對還原劑DTT敏感，我們猜想是否將omp2突變之後也會有跟dsbA突變之後有同樣的改變，所以我們將omp2利用基因插入的方式，使omp2突變，克雷白氏肺炎桿菌omp2突變株對還原劑DTT也一樣敏感，且鹼性磷酸酵素與脂酵素活性都明顯下降，並且突變株無法正常的生活在含有三梨醇微量培養基上。這些外表型的變化和dsbA突變很像，暗示著omp2可能和dsbA在生理上有同樣的功能。

Klebsiella pneumoniae is an opportunistic bacterium. It is a common hospital-acquired pathogen, causing urinary tract infections, nosocomial pneumonia, and intraabdominal infections. In Taiwan, K. pneumoniae is a significant cause of hepatic abscess. However, in western countries, liver abscess is usually a polymicrobial infection and agents of liver abscess are Escherichia coli, streptococci, and anaerobic bacteria. Most of the patients with K. pneumoniae liver abscess are diabetic and without biliary tract disease, in Taiwan. K. pneumoniae liver abscess has become an endemic disease in Taiwan in the past 2 decades.
In previous study, we have cloned a K. pneumoniae gene, omp2, whose product shows at amino acid sequence homology to the outer membrane protein, COM1, of Coxiella burnetii. The COM1 of Coxiella burnetii is thought an important factor for recognition of immune system and it has been developed as a vaccine for preventing cow from C. burnetii infection. To investigate whether the Omp2 is involved in immune response during K. p infection, Omp2 was used as a vaccine and the animal experiments of protective effect for preventing mice from K. p infection were performed. The result showed that Omp2 has no effect for protecting mice from K. p infection.
However, Omp2 has a motif that is homologous to active site of DsbA. In E. coli DsbA protein is the major oxidative catalyst in the periplasm and it affects disulfide bond formation of secretory and preplasmic proteins. dsbA mutants exhibit pleiotropic phenotypes. These phenotypes include reduction in levels of activity of the periplasmic enzyme alkaline phosphatase and lipase, and sensitivity to the reducing agent dithiothreitol (DTT). To invesigate whether omp2 mutant of K. p exhibit the same phenotypes to dsbA mutant of E. coli. omp2-defective was constructed by homologous recombisntion in this study and the phenotypes of omp2-mutant were determented. omp2 mutant is highly sensitive to DTT, shows reducing expression of alkaline phosphatase and lipase activity, and cannot grow well o sorbitol minimal plate. Theses phenotypes are very similar to those of dsbA mutant strains in other bacteria. The gene, omp2, may have the same physiological function of dsbA.

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