Pada Februari hingga Maret 2008 terjadi kejadian luar biasa muntah berak (diare) di Kabupaten Karangasem Bali. Tercatat sekitar 600 orang mengalami muntaber dan 5 orang meninggal dunia. Ini merupakan kejadian KLB muntaber pertama kali di Bali serta belum diketahui patogen penyebab diare tersebut. Tujuan penelitian ini adalah mengidentifikasi penyebab diare di Karangasem serta kemungkinan rantai penularannya. Penelusuran penyebab KLB dilakukan dengan menganalisis sampel air yang diambil dari sumber air umum, cubang/sumur penduduk, bahan makanan, serta rectal swab penderita dengan kombinasi teknik pemupukan kuman dan PCR spesifik dengan target gen pembentuk toksin pada Escherichia coli. Dengan melakukan kultur pada sampel makanan diperoleh bahwa 11 dari 21 sampel makanan positif mengandung E. coli. Dari sampel yang positif E. coli, 2 sampel yang diambil di rumah penderita muntaber terdeteksi gen pembentuk shiga like toxin tipe I dan II pada E. coli. Deteksi gen pengkode shiga like toxin tipe I juga terdeteksi pada penderita dan beberapa sampel air dari cubang penduduk. Hal ini menunjukkan bahwa E. coli pembentuk shiga ike toxin tipe I merupakan penyebab KLB di Karangasem. Lebih lanjut diperoleh bahwa pita shiga like toxin tipe I dan tipe II. E. coli strain Karangasem berbeda dengan strain EHEC sehingga strain Karangasem ini kemungkinan merupakan strain E. coli patogen baru yang terjadi akibat perubahan genetik pada E. coli pembentuk shiga like toxin yang ditemukan di Bali. In February to March 2008, Bali was shocked by the outbreak of diarrhea in Karangasem District, Bali. It was recorded that 600 people were having diarrhea and 5 people were died due to the disease. This outbreak was the first time happened in Bali and the causing pathogen was not yet identified. The aim of this study was to identify the causing pathogen of diarrhea in the outbreak case in Karangasem, as well as to identify the possible transmitting pathway. The tracking of outbreak cause was carried out by analyzing water sample taken from communal clean water source, private clean water reservoir, food sample, as well as rectal swab of the patient with the combination of pathogen enrichment technique and specific PCR with Escherichia coli as the target of toxin forming agent. Based on the culture growth from food samples, it was found that 11 from 21 samples were E. colipositive. From samples that E. colipositive, 2 sam-ples that were taken from patient’s house were detected a shiga like toxic forming gene, type I and II on the E. coli. The similar shiga like toxin forming gene type I was also detected on samples from patient and samples from water of private family cubang. This shows that E. colithat forms shiga like toxin type I was the diarrhea causing pathogen in this particular outbreak in Karangasem. Furthermore, it was found out that the ribbon formed by shiga like toxin type I and II differ from the strain of EHEC. Thus, it is possible that the strain found in Karangasem was a new strain of E. colipathogen due to genetic transformation on shiga like toxin forming E. coli that was found in Bali.